Add missing controller guidelines

[mcmire]

We have some missing controller guidelines:

• State should be coherent at all times. If properties are intended to be used in conjunction with each other they should be consistent, and we should have runtime validation to ensure that this is always the case.
• Use withController or setupController pattern to set up controllers in tests.
• Don't mock the messenger in tests.

Although the following is a package guideline, we could consider repeating it in the controller guidelines because that is probably more likely the use case:

• New controller packages should always start at 0.0.0 (otherwise they run the risk of being accidentally published).