PERF: Make stylesheet max_file_mtime calculation more efficient (PR #12876)

Plugins always store their stylesheets under /assets/stylesheets, so we can make the glob pattern much more specific. In my local development environment, this increases the speed of Stylesheet::Manager.max_file_mtime from ~65ms to ~3ms (20x faster). This significantly improves stylesheet regeneration time, and the responsiveness of the theme admin UI.

Note that this will have negligible effect in production, because in production the value of max_file_mtime is aggressively cached.