Add a "did you mean <>?" fix to UnknownRule

[a-frantz]

Upgrading discussion #106 to an issue, as I consider it good-to-go.

This will almost definitely include adding a new dependency to the workspace. There are some spellcheck-type crates out there and we should pull a solution off the shelf. If you're not sure which crate to pick, drop a comment and one of us will help you find the right choice!