Nix dev shell: upgrade fourmolu-0.18.0

[carbolymer]

Changelog

- description: |
    Nix dev shell: upgrade fourmolu-0.18.0
# 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

Fourmolu 0.18 supports import grouping but there's no column-wise alignment yet, so we're still keeping stylish haskell around for formatting imports.

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