feat: add CssContain, CssWillChange, CssTouchAction, CssUserSelect, CssHyphens

[kevin-sakemaer]

Summary

• Add CssContain sealed class for contain property with keyword shortcuts (none, strict, content), individual keywords (size, layout, style, paint), and .flags() factory for composing containment values
• Add CssWillChange sealed class for will-change property with auto, scrollPosition, contents keywords and .properties() factory for CSS property names
• Add CssTouchAction sealed class for touch-action property with standalone keywords (auto, none, manipulation), combinable keywords (panX, panLeft, panRight, panY, panUp, panDown, pinchZoom), and .combine() factory
• Add CssUserSelect sealed class for user-select property with none, auto, text, all, contain keywords\n- Add CssHyphens sealed class for hyphens property with none, manual, auto keywords
• All classes include .variable(), .raw(), and .global() escape hatches
• All integrated into Style.typed constructor\n- 100% test coverage on new code

Closes #86, closes #87, closes #88, closes #89, closes #90

Test plan

• All 581 tests pass
• New test files cover keywords, variable, raw, all 5 globals, and Style.typed integration for each property
• dart format applied
• Coverage verified (100% on new code)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!