Remove stylish haskell. Use only fourmolu for formatting.

[carbolymer]

Changelog

- description: |
    Remove stylish haskell. Use only fourmolu for formatting.
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
   - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

This PR changes our formatting method to just use fourmolu instead of fourmolu + stylish-haskell. Import grouping settings were copied 1-1 from stylsh settings.

Checklist

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

[smelc]

We could take this occasion to remove this loop that rolls only once now, but fine to leave it.

[smelc]

Nice 👍