Map `font-weight` web feature

[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.

Feature: "font-weight" Reference: https://github.com/web-platform-dx/web-features/blob/main/features/font-weight.yml