Set Fourmolu as formatter

[palas]

Changelog

- description: |
    Added Fourmolu to dev shell, to haskell-style-lint hook script, and modified stylish-haskell action to use Fourmolu instead
  type:
  - maintenance    # not directly related to the code

Context

We don't want to have to worry about formatting .hs files, and we want to be consistent. This PR tries to address this by establishing and enforcing a strong formatter (Formoulu).

How to trust this PR

Probably check that the hook works for you. Also that you are happy with the changes and configuration, and that the CI passes.

Checklist

• [x] Commit sequence broadly makes sense and commits have useful messages
• [x] New tests are added if needed and existing tests are updated. See Running tests for more details
• [x] Self-reviewed the diff