Consider diagnostic.code when generating flymake diagnostics

Consider diagnostic.code when generating flymake diagnostics

Not sure this will please everybody, can almost guess someone is going
to ask for a custom switch.

Instead this info (and the source) should be passed on to Flymake.
That's where the custom switch for controlling formatting of
diagnostic messages should exist.  But that's too much work right now.

* eglot.el (eglot-handle-notification): Consider Diagnostic.code.