Implement Custom Transformers (#4202)
The final part of moving the `next-*` crates to Next.js is to allow them
to use their own custom JS Transformers.
This was a bit of a headache. Our `turbo/nextpack` setup just doesn't
work with coc.nvim, we're midway through an SWC upgrade across our
repos, and I somehow `git reset --hard` the &*#(@$ changes while
working.
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Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
