Add the Kotlin 2.4.0 compiler plugin variant (component registrar,
IR compatibility shims, and version-specific utilities), bundle the
2.4.0 compiler dependencies, and update the build wiring, supported
version metadata and the too-new diagnostic bound.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
We are no longer bound to the platform-specific directories, so simplify the test organization.
If you don't want this change, just skip merging this PR. It's purely optional.
I kept the platform-specific directories around under `kotlin`,
but you could also easily merge all these together if you find them unhelpful.
I'll leave that change to you.
