Analyze use of `@layer` polyfill

[bartveneman]

via @romainmenke https://bsky.app/profile/romainmenke.bsky.social/post/3mc3mdv5iwk2s

Not sure if to file this under atrules.layer.polyfills or selectors.layerPolyfills.{total,totalUnique,unique,uniquenessRatio,ratio}. Leaning towards the latter.