[mlir-vscode] Add better resolution for server file paths

[mlir-vscode] Add better resolution for server file paths

We currently require that server paths are full paths, which is
fairly inconvenient for a myriad of reasons. This commit
attempts to resolve a given server path with the current workspace.

This has a nice additional affect that we can now actually have
default server paths. This means that mlir-lsp-server and
mlir-pdll-lsp-server can be transparently picked up from
build directories (i.e. generally no need for upstream users to
configure the extension).

Fixes #54627

Differential Revision: https://reviews.llvm.org/D122792