Introduce fourmolu (#1500)

Fixes #1499

# PR description

This PR introduces `fourmolu` as an automatic code formatter to replace
`stylish-haskell`

Beyond the actual reformatting, I've introduced the following changes:

- add `fourmolu` to the Nix shell
- add the `scripts/ci/run-fourmolu.sh` scripts, which is based on the
previously used script for `stylish-haskell`
- add a Hydra job to check the project with `fourmolu` (and pushed a
dummy commit to check that the CI rejects improperly styled code)
- add The Big Reformatting Commit to `.git-blame-ignore-revs` and
checked that the blame interface on GitHub respects it
- with a somewhat heavy heart, butchered the style guide. The
layout-related sections are removed as the layout is now enforced by
`fourmolu`.

Additionally, I've enabled `ImportQualifiedPost` in all cabal files,
alongside with the `-Wprepositive-qualified-module` warning. This
extension and warning could be removed once (if?) we transition to
`GHC2021` as the default language.

Author: geo2a

.git-blame-ignore-revs

@@ -0,0 +1,9 @@
+### NOTE
+# Run `git config blame.ignoreRevsFile .git-blame-ignore-revs`
+# from the repository's root to tell `git blame` to ignore
+# the commits below.
+
+# 2025
+########################################
+# Format with fourmolu
+70c3258667611a12f7fa8f85b5591830c533d296

CONTRIBUTING.md

@@ -203,19 +203,19 @@ and improvements are always welcome.
 
 ## Formatting the code
 
-We use `stylish-haskell` 0.14.6.0 for Haskell code formatting.
+We use `fourmolu` for Haskell code formatting. See [tools.nix](./nix/tools.nix) for the `fourmolu` version used in CI.
 
 Either enable editor integration or call the script used by CI itself:
 
 ```bash
-./scripts/ci/run-stylish.sh
+./scripts/ci/run-fourmolu.sh
 ```
 
 When using Nix, you can use the following command, which will build and use
-the right version of `stylish-haskell`.
+the right version of `fourmolu`.
 
 ```bash
-nix develop -c ./scripts/ci/run-stylish.sh
+nix develop -c ./scripts/ci/run-fourmolu.sh
 ```
 
 # Generating documentation and setting up hoogle