Switch from treefmt -> pre-commit

[srid]

I feel that the quality of treemt has lately declined. For eg., recent versions started dumping a bunch of needless noise (why is it now trying to format all files in repo?):

which then required more configuration in Nix to fix:

d8c7e63

And this unnecessarily extra config will vary across projects, depending on the files they have.

---

We strictly don't need treefmt though (besides, not everyone is allowed to contribute) because that's what pre-commit run -a is for.

To that end, this PR gets rid of treefmt and switches to https://github.com/cachix/git-hooks.nix (the Nix module for https://pre-commit.com/).

Functionality more or less remains the same - except for losing just formatter (which can be added to git-hooks.nix) and of course gaining git pre-commit hooks setup in devShell.

[srid]

Flake check is added and built on CI:

2024-11-19T00:28:21.3833522Z pre-commit-run> cabal-fmt................................................................Passed
2024-11-19T00:28:21.4654726Z pre-commit-run> fourmolu.................................................................Passed
2024-11-19T00:28:21.8773274Z pre-commit-run> hlint....................................................................Passed
2024-11-19T00:28:21.8875114Z pre-commit-run> nixpkgs-fmt..............................................................Passed