This PR builds on top of #14365 and adds a few more changes we discussed
during a sync on the latter PR:

- We now split `plugin-api.ts` into two files and moved it into
`compat/`. One file is now defining the comat plugin API only where as
another file deals with the compat hook.
- The walk for `@plugin` and `@config` is now happening inside the
compat hook.

The one remaining work item is to change the `loadPlugin` and
`loadConfig` APIs to a more unified `resolveModule` one that does not
care on what we try to load it for. I suggest we should make this change
at the same time we start working on finalizing the `tailwindcss` APIs,
since a lot of things will have to be rethought then anyways.

+ add codemod for migrating `@apply`

Copied some utils from `@tailwindcss/cli`. Initially referenced them,
but we also rely on some dependencies that we have to install.

One solution is to move it to an internal package and import it so that
tsup can inline everything. This also has the same 'we need the correct
dependencies' problem.

Another solution is to create a proper `@tailwindcss/cli-utils` (or
similar) package that we can import as a dependency. However, this one
will require us to actually publish the package, and give it a proper
name.