Manage folder changes

I tried in #4 to manage folder deletion/renaming, however, I didn't manage to register correctly the [`onDidChangeWorkspaceFolders`](https://microsoft.github.io/language-server-protocol/specifications/specification-3-15/#workspace_didChangeWorkspaceFolders) notification handler. 

The quick workaround for now is to simply reload the window but it would be convenient to not have to.