[mlir] Add the Emacs support for MLIR's LSP server

In order to use the MLIR LSP server in Emacs, first the `mlir-lsp-client.el`
has to be loaded via elisp and then, one should call `lsp-mlir-setup` function
to setup the LSP client. After that simply calling the `lsp` function while
the `mlir-mode` is active with result in finding the language server (default
to `mlir-lsp-server`) via the `lsp-mlir-server-executable` customization variable
and connecting to it by the LSP library. Users who use MLIR's language server
library to create their own server can simply set the variable `lsp-mlir-server-executable`
to point to their own implementation executable.

Reviewed By: tschuett

Differential Revision: https://reviews.llvm.org/D123002