Show LSP diagnostic errors in `Project Errors`

[angelozerr]

Some language servers (ex: our Qute language servers) can report errors for closed document. It would be nice if we could see those erros in the project errors:

vcode provides this feature and can show all errors in the error view and for Qute for instance you can see all errors of all Qute templates (closed and opened):

@ia3andy is it a feature that you would like to have?

I think this feature should be customized per language server definition and we should provide an API to know if LSP diagnostics must be displayed in Project Errors or not.