[mlir][Tablegen-LSP] Add support for a basic TableGen language server

This follows the same general structure of the MLIR and PDLL language
servers. This commits adds the basic functionality for setting up the server,
and initially only supports providing diagnostics. Followon commits will
build out more comprehensive behavior.

Realistically this should eventually live in llvm/, but building in MLIR is an easier
initial step given that:
* All of the necessary LSP functionality is already here
* It allows for proving out useful language features (e.g. compilation databases)
  without affecting wider scale tablegen users
* MLIR has a vscode extension that can immediately take advantage of it

Differential Revision: https://reviews.llvm.org/D125440