helix-syntax mostly existed for the sake of the build task which
checks and compiles the submodules. Since we won't be relying on
that process anymore, it doesn't end up making much sense to have
a very thin crate just for some functions that we could port to
helix-core.
The remaining build-related code is moved to helix-term which will
be able to provide grammar builds through the --build-grammars CLI
flag.
