Refactor XAML SourceGen TypeConverters

[jfversluis]

Moves the known TypeConverters to each their own file and class instead of a big bunch of methods.

Additionally optimizes the ColorTypeConverter to not just call Parse() but first try if we can produce more optimized code and some other minor things.

[Copilot]

Pull Request Overview

This PR refactors the XAML source generation type converters by moving from a large monolithic KnownTypeConverters class with static methods to a cleaner, more maintainable architecture with individual converter classes implementing a common interface.

• Introduces a new ISGTypeConverter interface and TypeConverterRegistry for managing converters
• Breaks down the large KnownTypeConverters class into individual converter classes for each type
• Updates the registration system to use the new registry-based approach with better organization

Reviewed Changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
TypeConverterRegistry.cs	New registry class that manages converter registration and lookup using a dictionary-based approach
ISGTypeConverter.cs	New interface defining the contract for source generation type converters
Individual converter files	Separate classes for each type converter (Color, Rect, Point, Thickness, etc.) implementing the new interface
SourceGenContextExtensions.cs	Extension methods for diagnostic reporting functionality
NodeSGExtensions.cs	Updated to use the new registry-based converter system
KnownTypeConverters.cs	Removed the large monolithic converter class

[StephaneDelcroix]

good to go as soon as it builds (see errors)