feat: simple design system library

[Kunal22shah]

GitHub Issue (If applicable): closes https://github.com/unoplatform/uno.themes-private/issues/31

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting)
• Refactoring (no functional changes, no api changes)
• Build or CI related changes
• Documentation content changes
• Other... Please describe:

Description

PR Checklist

Please check if your PR fulfills the following requirements:

• Commits must be following the Conventional Commits specification
• Tested the changes where applicable:
  • UWP
  • iOS
  • Android
  • WASM
  • MacOS
• Updated the documentation as needed:
  • General Doc Update
  • material-controls-styles.md
  • cupertino-controls-styles.md
  • lightweight-styling.md (LightWeight Styling Resource Keys)
• Contains No breaking changes

  If the pull request contains breaking changes, commit message must contain a detailed description of the action to take for the consumer of this library. As explained by the Conventional Commits specification

Other information

Internal Issue (If applicable):

[Copilot]

Pull request overview

This PR introduces a new "Simple" design system library to the Uno.Themes repository, adding a third design option alongside the existing Material and Cupertino themes. The Simple design system provides a minimalist, neutral aesthetic with comprehensive styling for common UI controls.

Changes:

• Added complete Simple design system library (Uno.Simple.WinUI) with color palette, fonts, and control styles
• Integrated Simple theme support into the samples application with dedicated sample pages
• Created markup extensions library (Uno.Simple.WinUI.Markup) for C# markup support

Reviewed changes

Copilot reviewed 43 out of 44 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/library/Uno.Simple.WinUI/*	Core Simple design system library with styles, colors, and theme infrastructure
src/samples/UWP/Uno.Themes.Samples.Shared/*	Sample pages and integration for demonstrating Simple controls
src/library/Uno.Simple.WinUI.Markup/*	C# markup extensions for Simple theme
Uno.Themes.sln	Solution file updated to include new Simple projects
src/library/Uno.Themes/themes-common.props	Added InternalsVisibleTo for Simple library

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[Copilot]

Material theme resources are commented out in the samples application. If this is intentional for testing the Simple theme in isolation, it should be documented with a comment explaining why. Otherwise, both themes should be available in the samples app for comparison.