Replacing ILanguageServerProtocol with AbstractRequestHandlerProvider.
Refactoring common Extensions method code into generic helpers.

Added XAML LSP shims by introducing ILanguageServerProtocol and adding an optional Language parameter to the ExportLspMethodHandlerAttribute so that XAML can export its own LSP handlers.
Added a few extension methods to make dealing with TextDocuments easier.

This replaces our existing "GetDocumentFromURI" helper with one that
instead takes the full TextDocumentIdentifier, and uses the context
information to figure out which document to use in the case of
multi-targeting or linked documents.