Map `gap-decorations` web-feature

[stalgiag]

Feature: gap-decorations Reference: https://github.com/web-platform-dx/web-features/blob/main/features/gap-decorations.yml

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Notable exclusions

1. Forced Colors Mode tests
   • forced-colors-mode/forced-colors-mode-55.html
   • forced-colors-mode/forced-colors-mode-56.html
   • These tests focus on forced-colors behavior using gap-decorations properties, rather than the gap decorations feature itself.

[wpt-pr-bot]

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