[mlir][PDLL] Add an initial language server for PDLL

This commits adds a basic language server for PDLL to enable providing
language features in IDEs such as VSCode. This initial commit only
adds support for tracking definitions, references, and diagnostics, but
followup commits will build upon this to provide more significant behavior.

In addition to the server, this commit also updates mlir-vscode to support
the PDLL language and invoke the server.

Differential Revision: https://reviews.llvm.org/D121541