I am just a simple developer trying to send these cancellation tokens downstream where they belong

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.

None of this actually depended on the editor in any meaningful way.

- Razor has implemented and inserted the "proper" handling of diagnostic URIs. Prior to this they would prefix file paths with a `/`. That's no longer true.
- Did not remove the entire method because it still has value in converting file paths into Uris and looks to be used by more than just the `InProcLnaguageServer` diagnostic path.