Add Nix packaging support

[Aspiand]

Adds a flake.nix to build and install the extension natively on NixOS.

Usage

hanabi = {
  url = "github:jeffshee/gnome-ext-hanabi";
  inputs.nixpkgs.follows = "nixpkgs";
};

Then in Home Manager:

let hanabi = inputs.hanabi.packages.${pkgs.stdenv.hostPlatform.system}.default;
in {
  home.packages = [ hanabi ];
  dconf.settings."org/gnome/shell".enabled-extensions = [
    hanabi.passthru.extensionUuid
  ];
}

See docs/distro/nixos.md for full details.

[Krutonium]

I'm trying this out, but for whatever reason the best I can get is a black background. I configured a video as well as a folder of images, and even toggled on Clapper, but nothing is actually making it render anything.