Clean up compiler libs

[cknitt]

There are currently two different compiler libs in the repo:

1. res_syntax/compiler-libs-406: created by @cristianoc for the syntax repo in https://github.com/rescript-lang/rescript-compiler/pull/5820/files#diff-ae689c9d78337a2d2fdc22773dbd60e8c74cac71ac957d4d6a16a2a9d6fb7c93
2. jscomp/ml: the much more heavily modified one from the compiler repo itself

Currently, the syntax lib, cli and benchmarks (all in res_syntax) are built with 1.
For the compiler build, the syntax sources are copied to the napkin target and compiled with 2.

We should resolve this so that there is only a single version of the compiler libs. But ideally this single version should not drag in too many dependencies (e.g. ml depends on ext which depends on C stubs) so that the ReScript parser can easily be included in other projects.