Update Source Generators to use new Incremental Source Generator API

[Copilot]

This PR updates both XAMLTest.Generator and XAMLTest.UnitTestGenerator from the legacy ISourceGenerator interface to the modern IIncrementalGenerator interface introduced in Roslyn 4.0.

Changes Made

ElementGenerator.cs

• Interface Update: Changed from ISourceGenerator to IIncrementalGenerator
• Initialization: Replaced Initialize(GeneratorInitializationContext) with Initialize(IncrementalGeneratorInitializationContext)
• Execution Model: Replaced Execute(GeneratorExecutionContext) with RegisterSourceOutput using incremental value providers
• Syntax Processing: Converted ISyntaxContextReceiver pattern to SyntaxProvider.CreateSyntaxProvider() with predicate and transform functions
• Code Organization: Moved helper methods and constants from SyntaxReceiver class to static methods in ElementGenerator

UnitTestGenerator.cs

• Applied the same incremental generator conversion pattern
• Restructured static helper methods at class level
• Maintained all existing business logic and output generation patterns

Benefits

The incremental source generator API provides several advantages:

• Better Performance: Only regenerates code when relevant inputs change through intelligent caching
• Improved IDE Experience: Better integration with IntelliSense and real-time diagnostics
• Composable Pipeline: More modular and testable generator architecture
• Future-Proof: Aligns with Microsoft's recommended approach for source generators

Verification

✅ Both generators build successfully with zero warnings
✅ Generated output remains identical to previous implementation
✅ Test compilation confirms generators produce expected files
✅ All existing functionality preserved

The migration maintains 100% backward compatibility while modernizing the codebase to use current best practices.

Fixes #280.

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