Integrate with `nix fmt`

[Jairo Llopis]

As you can see in the manual, there's a nix fmt option for flakes. Flakes can indicate what program should be used for formatting code. Flakes that use this project will usually be formatted using it, so I think it would be helpful to provide formatter configuration out of the box when using the flake template. Also documenting how to use it for projects that already have a flake.

[Karol Ochman-Milarski]

This is much needed option, because - while the corrected formatting gets applied to staged changes - there's no way to apply formatting to all tracked files.

EDIT: its there at pre-commit run --all-files. The integration with nix fmt would be cool nevertheless.

[Domen Kožar]

If someone makes a PR that would be great!