This new rubocop rule enforces that all resolvers declare their type.

This is important for several reasons:

- developers can read a resolver and know what it does, by understanding
  what it is meant to return.
- we can enable better testing of resolvers, by creating fields from
  resolvers using `Resolver.field_options`.
- Resolvers become the SSOT for field metadata, enabling us to eliminate
  repetition and boilerplate.