[XSG] Simplify ColorConverter

[simonrozsival]

Description of Change

Color.TryParse already implements all the supported formats, no need to reinvent the wheel. Also, the compiled value uses the most optimal constructor - all floats.

This PR:

• moves parsing and conversion logic from Color to ColorUtils so that it can be easily shared between Graphics and SourceGen projects
• simplifies the source gen color converter by using ColorUtils + instead of keeping its own list of known colors, it scans the static fields of the Colors class

[Copilot]

Pull Request Overview

This PR simplifies the ColorConverter source generator by leveraging the existing Color.TryParse method instead of maintaining custom parsing logic. The change eliminates redundant code that was reimplementing color format support.

• Removes 60+ lines of custom color parsing logic including regex patterns and named color constants
• Replaces multiple parsing approaches with a single Color.TryParse call
• Updates generated code to use optimized float constructor with invariant formatting

[StephaneDelcroix]

nope

we don't want to have dependencies. can we replace that with a Compile-Include ?

[simonrozsival]

I tried to do this real quick and it turns out to be challenging because Color.cs as is pulls in a bunch of other classes and one of them requires MathF which is not netstandard2.0. I will look into this tomorrow and I'll see if I can easily move the parsing logic to a separate file which could be shared between the two projects.