feat(migration): Add SASS to PostCSS migration evaluation and script

[Copilot]

• Explore repository structure and understand current SCSS setup
• Analyze current SCSS features usage (mixins, functions, placeholders, @if/@each/@else)
• Create migration specification document with detailed analysis (docs/migration/sass-to-postcss.md)
• Create migration script prototype in the migration package
• Add tests for the new migration script
• Run tests to verify migration script works correctly
• Document the two-output PostCSS build approach (chromium vs legacy) - documented in spec
• Address code review feedback:
  • Fixed dollar sign escaping in mixin parameter replacement
  • Added single-quote support for @use and @forward imports
  • Fixed comment regex to avoid URL issues
  • Enhanced partial import patterns for various path formats
  • Updated test assertions to use toBeDefined() matcher
• Add postcss-each plugin for Sass @each loop replacement

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[changeset-bot]

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Latest commit: a95e451

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[mfranzke]

@copilot for Sass @each replacement, we could use postcss-each PostCSS plugin.

[Copilot]

@copilot for Sass @each replacement, we could use postcss-each PostCSS plugin.

Added postcss-each to the Core Plugins list and updated documentation to recommend it as the primary approach for @each loop replacement. Commit a95e451.