Use local user css cache for apply

[thecrypticace]

Background

The PR #6580 caused any file using apply to create a new context. This meant that any file that used apply would also carry along the entire content dependency tree. Updating any file covered by content would cause these files to recompile. The original cause of the problem that #6580 was fixing turned out to be a cache invalidation issue. Since user CSS was added to the context things could end up incorrectly shared across files when it comes to apply. However, this would only happen once and future updates would not invalidate the cache. This would manifest as an error about the applied classes being missing in some cases.

Why did it seem to fix the original problem?

By creating a new context for any file containing apply it caused the context to get re-created every time that file was saved. Because of this the CSS was re-scanned and added to the internal user CSS cache that we had on the context.

What are we going to do instead

1. I've partially reverted Allow all classes for @apply #6580 to ensure that contexts are reused when using @apply. The reason this is safe is because the config can affect apply but content files cannot.
2. I've added an additional cache for apply to look at the root postcss node that's being passed in instead of internally storing user CSS as individual tailwind plugins.

This is still a work in progress. Some of the code here needs to be cleaned up first.

Fixes #7480

[RobinMalfait]

Functionally it seems to work great, nice!
I do have some questions / nitpicks.

[hivokas]

@thecrypticace thanks for working on that!