feature: nix lib to generate custom catppuccin themes

Carburetor should be able to house something like this: ```nix 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