Provide GHC 9.0+ plugin for beam error messages

According to https://well-typed.com/blog/2021/08/the-new-ghc-diagnostic-infrastructure/, GHC will soon have the ability to load 'diagnostic messages' plugins that allow libraries to transform GHC error messages. Beam is well-known for bad error messages, and this is due to its heavy usage of type families and type classes. Nevertheless, the errors all follow similar patterns, especially for newcomers. It would be nice to develop a plugin that at least offered helpful suggestions based on the issues that have been filed against beam. This would help newcomers and old-timers alike.