CLaSP: Allow handlers to register multiple methods

[ryanbrandenburg]

Currently CLaSP doesn't support registering a single class for more than one method. It might be nice in some cases to allow very similar or noop endpoints to share a class (think didOpen/didChange, or the ranged and document-wide versions of a request).

[ryanbrandenburg]

Another example of combinations that could be made is RazorTranslateDiagnosticsEndpoint and RazorPullDiagnosticsEndpoint.

[ryanbrandenburg]

Of note, the error message given by this scenario currently is totally unhelpful unless you go code spelunking, so we need to fix that at minimum.

Also at least one implementing team ran into this so we might want to re-consider having it on the backlog.