Add Nix flake, package, and NixOS module

[micahsoftdotexe]

This adds nixos support for this fork of the abcde application. I had been using the original application in nixpkgs but wanted to use some of the features in this branch. Note: just wanted to disclose that I did have AI assist in the making of this code but have done manual review.

[poddmo]

Wow, thanks for the contribution. I'd never heard of flakes before. I'm very happy to have package support for broader distributions. It will take me a while to run up a VM and test this for myself. It would be good to get feedback from another NixOS user that is experienced.

Off the top of my head I'm thinking:

• the NixOS specific stuff should go into a dedicated folder called nixos
• create a readme-nixos.md for the documentation and link to it from the main readme.md
  Does that sound reasonable to you?
  Thanks again.

[micahsoftdotexe]

That sounds pretty reasonable and I will get on that! The only thing I would bring up is for single-flake repos , the flake.nix and the flake.lock by convention are usually root of the repo. There is a way to have the flake in a subdirectory by adding the ?dir= syntax when you want to reference the flake. For instance, this is the code that I am using in my current nixos config, to currently target the fork I made for the changes, this is what I included in the flake.nix for my nixos repo:

 abcde = {
      url = "github:micahsoftdotexe/abcde/feature/nix-flake";
      inputs.nixpkgs.follows = "nixpkgs";
    };

This is what it would be if we placed the flake files into the nixos directory:

abcde = {
  url = "github:micahsoftdotexe/abcde/feature/nix-flake?dir=nixos";
  inputs.nixpkgs.follows = "nixpkgs";
};

sort of the same would apply when this would be merged into this repo except the url would change and I wouldn't have to specify a branch since it will be in the default branch
url = "github:poddmo/abcde"; vs url = "github:poddmo/abcde?dir=nixos";

I can make that change although I haven't run into the flake.nix and the flake.lock being put into a subdirectory but I am perfectly willing to add both to the subdirectory if necessary! I will move the others and work on the README in the meantime! 😃