Group common scripts

Concat can be a little overzealous in cases where a script or style is enqueued conditionally. Say a script is used on one page. Ideally we wouldn't serve a different Javascript bundle on that page that is the same as every other page with the one extra script added. The bundle that loads on every other page is most likely already cached in the user's browser. We really just need to serve the additional script separately (or maybe inline?).

Review the plugin's concatenation flow to determine how conditionally enqueued scripts and styles are currently grouped. Define how shared bundles, page-specific resources, caching, and optional inlining should behave, then verify the result against pages with and without the additional resource.

Written by the indexing model from the issue text.