Make Compiler a stdlib
This is a further extension to #56128 to make the compiler into a proper
stdlib, useable outside of `Base` as `using Compiler` in the same way
that `JuliaSyntax` works already. There's a few remaining questions
around how loading works, but mechanically, this PR is complete.
For those remaining questions, I'll probably put up a separate PR
that would migrate JuliaSyntax to it and then simply adopt that
here once we've figured out the correct semantics.
