[mlir-vscode] Add better support for multiple workspace folders

We currently only launch one set of language clients when starting the extension,
but this has the unfortunate effect of applying the same settings to all workspace
folders. This commit adds support for multiple workspace folders by launching
a server for each folder in the workspace. This allows for having different servers
for different workspace folders, e.g. when there are multiple MLIR projects in
the same workspace.

Differential Revision: https://reviews.llvm.org/D122793