Handle unsupported file URI schemes more reliably

[daviwil]

VS Code keeps adding new file URI schemes that somehow end up getting passed through to our language server (usually a bug on their end). The short-term fix is always to add the unexpected URI to Workspace.IsPathInMemory but I think that since this keeps happening, we need an official mechanism to handle any unexpected URI scheme.

Fixing this will require more robust checks in all of the code that interacts with the Workspace so I'm going to hold off on this work until at least 0.9.1.