Add fourmolu as a formatter

[martijnbastiaan]

See discussion at clash-lang/clash-protocols#79. Ideally this would be easy to integrate into git hooks for people that want that. Like for clash-protocols: let's follow fourmolu's opinions as much as possible. The point is to be consistent, not to impose yet another style to get used to.

[rowanG077]

The format.sh script in diff mode is basically designed to be be used as the git pre-commit. I did play around with it a little bit on the protocols PR but in the end did not introduce it because commit a pre-commit hook requires everyone to configure it manually. Personally I configure my editor to run the formatter on save.