feature: nix lib to generate custom catppuccin themes

[ozwaldorf]

Carburetor should be able to house something like this:

fooThemeOverlay = inputs.carburetor.lib.mkCustomTheme {
  name = "foo";
  colors = {
    mocha = { ... };
  };
  flavor_names = {
    mocha = "regular";
  };
};

pkgs = import nixpkgs {
  inherit system;
  overlays = [ fooThemeOverlay ];
};

Which would provide the complete package set overridden and patched to the inputs. After this, the nix flake for the carburetor theme should just parse src/whiskers.json for the color overrides, and output the overlay and packages