Add some easy TcRn messages

This MR capitalises on !5509 (closed) and starts adding some very simple-to-port TcRnMessages, to start making concrete steps towards #18516. It also gives the audience a sneak peek of the final API.

Me and @rae might need to discuss how to tame the zoo of logging functions we have inside GHC.Tc.Utils.Monad: at the moment I have added two provisional functions, addDetailedDiagnostic and addTcRnDiagnostic to avoid creating another big MR, but obviously we want to use very few functions (ideally one or two). Suggestions about the best migration path welcome.

Surprisingly enough, we didn't have a test for the dodgy exports flag, so I added it.