Define guidelines for compiler messages

[adamconnelly]

At the moment the format of the messages for the compiler is defined here, but we should go a bit further and specify guidelines that all the messages should follow to make sure they're consistent, and also to make it easier for people to add new messages.

Some examples of what we might wanna think about are:

• How to structure each message (e.g. Problem, Potential Solution).
• The voice to use (e.g. "I couldn't find the 'User' type" vs "'User' could not be found").
• Making sure to include context information where possible (e.g. "'Username' uses the slist type" vs "slist should not be used".

There's a really interesting paper written about this here: https://www.hindawi.com/journals/ahci/2010/602570/