[WIP] perf: Brush change subscription

[MartinZikmund]

GitHub Issue (If applicable): closes #

Replaces #12234

PR Type

What kind of change does this PR introduce?

What is the current behavior?

What is the new behavior?

Copilot Summary

🤖 Generated by Copilot at 9032c90

This pull request improves the performance and memory usage of the brush system by adding a new feature to Brush.cs that allows subscribing to brush changes without creating new delegates or closures. It also simplifies and refactors the using directives and the brush change handling in TextBlock.cs and TextBox.cs. It moves some types to separate files for clarity and consistency. It updates the Brush.skia.cs and SolidColorBrush.cs files to use the new feature.

PR Checklist

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• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
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• Contains NO breaking changes
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Other information

Internal Issue (If applicable):

[MartinZikmund]

Closing in favor of #12896