Consider adding a code formatter

I've been using nixfmt in accordance with @manveru's preference since I started, but I've come to notice that not everyone seems to be doing this. This leads to larger diffs in commits than is really necessary. I can add a pre-commit hook to run a formatter, but I'd first like to gain a bit of feedback about which to use.

The only real contenders at this point are [nixfmt](https://github.com/serokell/nixfmt) and [nixpkgs-fmt](https://github.com/nix-community/nixpkgs-fmt). While I actually agree that `nixfmt` has nicer output atm, my vote would be for `nixpkgs-fmt` simply because it seems to be what nixpkgs will eventually use, and so it's more likely to not be abandoned.

/cc @jonringer @kreisys @johnalotoski @sevanspowell @cleverca22