Expose color palette as attributes

[musjj]

Expose a set of read-only attributes containing the chosen Catppuccin color palette.

Example usage:

{ config, ... }:
{
  my-color = config.catppuccin.palette.colors.base.hex;
}

This will help eliminate IFD for modules that require access to the color palette for various reasons. Users who wants to add personal touches/customizations to their themes will also benefit from this.

The workflow for the palette generation is meant to be run manually (via workflow_dispatch), because I don't think it needs frequent/regular updates. But I can change that if necessary.

I'm not confident with how the files are organized, but I've decided to create a separate file for the palette types because it got a bit long. Also, there might be a way to automatically generate the Nix types from the TypeScript types, but it's written manually by hand for now.

Fix: #734
Ref: #392