Plugin API to prepare for frontend implemented in julia

[c42f]

I thought I'd try to get the first logical part of #31954 in to make it easier to iterate on separating the frontend.

There's two commits here, the first is pure code movement. The second is @Keno's work to create jl_frontend_t, with some additional minor cleanups (eg, make the new jl_fl_* functions static).

[vtjnash]

It appears the first commit is simply renaming the file but leaving behind a couple functions? It seems like it might be much simpler just to move the remaining handful of lines to better places (jl_has_meta could go in codegen.cpp, since it's only needed there, and I think jl_copy_ast would normally live in rtutils.c now).

The second commits seems to combine several semi-unrelated things that all happened to be in ast.c previously, into one struct, for no apparent reason. I get that there's a plan to replace parsing, but I don't think we also plan to, or need to, make flisp initialization (init) and print (the Informational queries) pluggable via the same interface.

[Keno]

The point is not to have a perfect interface, but to do the minimal thing that separates out all the flisp parts into a separate object that can be replaced wholesale. The informational queries depend on constants defined by the parser so they of course need to be replaced along with the parsing proper. @c42f wants to work on replacing lowering also, so we may as well move all the flisp-dependent code behind this interface. We can of course make it a simpler or more streamlined interface in future refactorings.

[c42f]

Yes, the idea here is to get a first version in to make it easier to iterate on things (I realize the interface leaves some things to be desired)

Having said that, code movement is painful and disruptive to other PRs so it would be nice to do a good job of it. I'll look at ast.c in a bit more detail and see whether the remaining stuff can be moved elsewhere.

[c42f]

I don't think we're going to do this in quite this way. Will reopen a new version if I get back to this later.