Add embedded syntax highlighting for github-script and run blocks

[dncrews]

Context
This PR is intended as an extension of #570. If we merge this one, that work (and the extracted expression grammar work from #468) would land together.

This also provides a concrete implementation for the long-standing actions/github-script highlighting request in #194.

Context from #570

Is your feature request related to a problem? Please describe.
Syntax-highlighting regressions in GitHub Actions workflow expressions are hard to diagnose and easy to reintroduce. This codebase did not have testing around these, and there have been several errors and a few regressions dealt with and managed in Issues.

Describe the solution you'd like
This PR adds a small workflow syntax test harness, fixture-based expression coverage, and the expression grammar changes needed for those cases to pass.

The expression grammar changes in this PR are extracted from and based on prior work by @cdce8p in #468, including fixes for block if: handling, comment-related if behavior, multi-line inline expressions, and nested-curly-brace expression parsing.

My additions in this PR are the supporting work needed to land and maintain that logic in a smaller reviewable unit:

• focused Jest regression tests for the expression grammar
• YAML fixtures covering the affected expression cases
• lightweight syntax test utilities used by those tests
• documentation for grammar triage and fixture-based regression testing
• the small Jest import update in the existing secrets test for consistency with the current test style

Additional context
Shout-out to @cdce8p for the original expression grammar work in #468, which this PR builds on and extracts into a smaller reviewable unit.

This PR is the first half of a split from a larger change set. It intentionally contains only the expression grammar changes and the supporting regression-test/documentation work. Embedded syntax highlighting for github-script and run blocks will be submitted separately.

Is your feature request related to a problem? Please describe.
Workflow files can contain embedded languages in places like actions/github-script with.script blocks and run: blocks with explicit shell: values, but those regions are currently highlighted as plain YAML. That makes JavaScript, shell, PowerShell, batch, Python, and similar embedded code harder to read and review.

In particular, actions/github-script support has been explicitly requested before in #194.

Describe the solution you'd like
This PR adds embedded syntax highlighting for workflow script bodies by registering injection grammars for:

• actions/github-script with.script blocks
• run: blocks when the step has an explicit supported shell:

This builds on the expression grammar and test scaffolding work in #570. In particular, this PR adds:

• grammar registrations for embedded workflow injections
• an embedded JavaScript grammar for actions/github-script
• embedded grammars for explicit run shells such as bash, PowerShell, batch, Python, and Node
• focused fixture-based regression tests for those embedding rules
• the additional syntax test utilities and documentation updates needed for embedded-language cases

Additional context
This PR is the second half of a split from a larger change set. It complements #570 by extending the workflow syntax work from expression handling into embedded-language highlighting.

Validation:

• npm test

[Copilot]

Pull request overview

This PR extends the GitHub Actions workflow TextMate grammars to support embedded-language syntax highlighting (notably actions/github-script and run: blocks with explicit shell:), and adds a lightweight fixture-based Jest regression suite to prevent highlighting regressions.

Changes:

• Register new injection grammars and embedded-language mappings for actions/github-script (with.script) and run: blocks with explicit supported shell: values.
• Add new TextMate grammar files for github-script embeddings and run+shell embeddings, plus updates to the expressions grammar for improved if: handling.
• Introduce fixture-based Jest tests and shared utilities/documentation for validating embedded and expression grammar behavior.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/workflow/syntax/syntax-test-utils.ts	Adds shared helpers for loading grammars/fixtures and asserting embedded/expression boundaries.
src/workflow/syntax/syntax-embeddings.test.ts	Adds regression tests for github-script and run/shell embedding detection.
src/workflow/syntax/expression-syntax.test.ts	Adds regression tests around expression parsing edge cases and if: tokenization.
src/workflow/syntax/README.md	Documents triage process and how to add fixture-based grammar regression tests.
src/workflow/syntax/fixtures/run-shell-embedded.yml	Fixture covering supported shell:-driven run: embeddings (pwsh/bash/node).
src/workflow/syntax/fixtures/run-shell-edge-cases.yml	Fixture covering non-block run: and comment/quoting edge cases for shell:.
src/workflow/syntax/fixtures/inline-multiple-expressions.yml	Fixture for multiple ${{ }} occurrences on one line.
src/workflow/syntax/fixtures/if-inline-edge-cases.yml	Fixture for if: with # inside quotes and escaped quotes.
src/workflow/syntax/fixtures/if-comment-after-string.yml	Fixture for trailing YAML comment handling after quoted strings in if:.
src/workflow/syntax/fixtures/if-block-expression.yml	Fixture for block-scalar `if:
src/workflow/syntax/fixtures/github-script-embedded.yml	Fixture for embedding JS only within actions/github-script `script:
src/workflow/syntax/fixtures/github-script-comments.yml	Fixture for comments on uses: and script: headers in github-script steps.
src/workflow/syntax/fixtures/expression-nested-braces.yml	Fixture for ${{ }} termination with }} inside quoted strings.
src/workflow/syntax/fixtures/expression-multiline.yml	Fixture for multi-line inline expression parsing.
package.json	Registers new injection grammars and maps embedded scopes to VS Code language IDs.
language/syntaxes/run-shell-embedded.tmLanguage.json	Implements run: block embeddings based on explicit shell: in the same step.
language/syntaxes/github-script-embedded.tmLanguage.json	Implements embedded JavaScript tokenization for actions/github-script with.script blocks.
language/syntaxes/expressions.tmGrammar.json	Updates expression injection patterns and adds operator patterns + improved if: handling.
language/README.md	Adds a pointer to the workflow syntax triage/testing documentation.
src/secrets/index.test.ts	Aligns test import style by explicitly importing Jest globals.

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[dncrews]

Hey @ericsciple:

First, I apologize in advance for the two direct tags on you (with almost identical text). The main reason for two incoming pings is that I wanted to allow the PRs to stand on their own as much as y'all want, and so I wanted to separate the discussion but let the team have full context. I normally try not to hit people up directly but since these two PRs (#570 & #571) touch on so many issues, there wasn't really a single thread for me to extend the conversation, and I wasn't quite sure how you would like me to proceed.

---

The original issue I wanted to discuss was embedded syntax highlighting. I found #194 that discussed that, so I figured I'd move ahead on that one.

In the process of building that, I wanted a bit more confidence in my changes, so I put in some testing components. That made the PR large enough that I didn't feel comfortable moving forward without splitting it into two:

• That PR (Add testing harness for grammar #570): the testing harness and the fixes contributed in Expression grammar improvements #468
• This PR (Add embedded syntax highlighting for github-script and run blocks #571): the embedded syntax highlighting