Implement custom Turbopack Next transformers (#47137)
Builds on https://github.com/vercel/turbo/pull/4202 to implement custom Next.js Transformers in Turbopack.
This is the final piece to moving the `next-*` crates to Next. While we've _technically_ moved everything, Turbopack didn't support running custom transformers. So we're actually stuck on the last version we cut before deleting the next crates, running the transformers that exist in the turbopack repo. With the new support, we're almost back to the tip of main branch (there's still some snafu with `swc_core` upgrading that I'm working on).
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
