add a NixOS module to nixpkgs

[Schweber]

First of all, thank you for your work and effort on this!

So far i haven't used flakes and am thus a bit hesitant about it. That's why i am wondering why you havent't added a niri module to NixOS (like the one for hyprland for example)?

I understand that using your flake provides more flexibility but i don't need that and would be happy with a "vanilla" module.

Would you consider adding one (while continuing your flake of course)?