Fix LSP document selectors for notebooks on Windows

[seeM]

Attempts to address #4006 where completions are empty for notebooks.

I haven't been able to validate the fix, but we did discover in https://github.com/posit-dev/positron/issues/3006#issuecomment-2124800883 that on Windows, the Uri.path attribute is not a valid file path e.g. /c:/Users/foo and that we should use Uri.fsPath instead. I also confirmed from the user's logs that the LSP server is starting up and the client is connecting correctly, so this seems promising.