[mlir][tblgen] Refact mlir-tblgen main into its own library

This has previously been done for `mlir-opt` and `mlir-reduce` and roughly the same approach has been done here.

The use case for having a separate library is that it is easier for downstream to make custom TableGen backends/executable that work on top of the utilities that are defined in `mlir/TableGen`.
The customization point here is the same one as for any upstream TableGen backends: One can add a new generator by simply creating a global instance of `mlir::GenRegistration`.

Differential Revision: https://reviews.llvm.org/D131112