Map `font-variant`, `font-variant-caps`, `font-variant-east-asian`, `font-variant-ligatures` and `font-variant-numeric` web features

[howard-e]

Hello, reviewers! As of 2025-12-05, the wpt-pr-bot is requesting reviews from code owners for changes to WEB_FEATURES.yml files. To learn more about the purpose of these files, check out this presentation from TPAC 2025, Annotating WPT to Surface the Status of the Platform.

Features:

• font-variant, "font-variant"
• font-variant-caps, "font-variant-caps"
• font-variant-east-asian, "font-variant-east-asian"
• font-variant-ligatures, "font-variant-ligatures"
• font-variant-numeric, "font-variant-numeric"