Add Nix for pre-commit dependencies

[berquist]

Added because I kept having problems with ruamel.yaml in my Python env, and this basically works.

[berquist]

I want to add the pre-commit config to Nix but that makes it basically impossible to use pre-commit for people who don't want to use Nix...thinking about that...

[berquist]

Also I need to add patch, coreutils, and diffutils explicitly to the flake. (They do seem to be loaded transitively already and not from my OS.)