[mlir:PDLL-LSP] Add a custom LSP command for viewing the output of PDLL

This commit adds a new PDLL specific LSP command, pdll.viewOutput, that
allows for viewing the intermediate outputs of a given PDLL file. The available
intermediate forms currently mirror those in mlir-pdll, namely: AST, MLIR, CPP.
This is extremely useful for a developer of PDLL, as it simplifies various testing,
and is also quite useful for users as they can easily view what is actually being
generated for their PDLL files.

This new command is added to the vscode client, and is available in the right
client context menu of PDLL files, or via the vscode command palette.

Differential Revision: https://reviews.llvm.org/D124783