LSP: LanguageClient `pullDiagnostics` request support

Hi again,

so far, integrating ruby-lsp works well already, however to support the issues highlighting feature we would need the Nova LanguageClient to support pullDiagnostics capabilities as well.

I already tired to check if that could be done via custom sendRequest() on document save/change, however they also need to be advertised as capabilities and the response needs to be handled and not only returned to the caller to populate the issues in the workspace files.

Hopefully this can be done given that similar functionality from publishDiagnostics is already implemented. Perhaps you can look into it.