Add flake-utils-plus

[kilianar]

flake-utils-plus exposes a library abstraction to generate NixOS flake configurations.

[cyntheticfox]

In the vein of #125, could we change the description to something clearer like "A lightweight library flake for painless NixOS flake configuration"? This makes its purpose clearer, and the "lightweight" part differentiates it from something like DevOS. (Maybe "library Nix flake"? "Nix library flake"? "Nix flake library"? Unsure what terminology I'd specifically use, but that's semantics)