Support subdirectory matching in WEB_FEATURES.yml

[foolip]

As suggested by @mustaqahmed in #56513 (comment), if we supported matching subdirectories in WEB_FEATURES.yml, it would be possible to have one WEB_FEATURES.yml file per directory in many cases. This would be more like https://github.com/tc39/test262/blob/main/WEB_FEATURES.yml.

cc @jcscottiii @ChrisC @jugglinmike