SourceKit-LSP prepares the currently active file for editor functionality and currently infers the currently active document from whichever received the last `TextDocumentRequest`.
If an editor is capable of doing so, it should be able to report the document that the user currently has focused so that SourceKit-LSP does not have to infer this information from other requests.
Also clean up some handling code for experimental capabilities.
4d00c9089b