[Refactor] Use declarative .find() in import-extractor resolution helpers

[gonzaloriestra]

What this PR does / why we need it

Refactors resolveJSImport and resolveRustModule in packages/cli-kit/src/public/node/import-extractor.ts by replacing imperative for...of loops with declarative Array.prototype.find() calls. Also replaces manual string slicing for extension extraction with extname from ./path.js.

How to test your changes?

CI

Checklist

• I have executed pnpm suppress-warnings from the root of the repository to record any new compiler warnings introduced by this PR.
• I have updated the documentation in /docs to reflect any user-facing changes in this PR.
• I have created a changeset using pnpm changeset if this PR contains user-facing changes or changes that should be published.
• I have added tests to cover my changes.
• I have run pnpm run lint and fixed any linting errors.
• I have verified that all tests pass locally.

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PR created automatically by Jules for task 11092848067801520960 started by @gonzaloriestra

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