Act as a Python game developer with 5+ years of experience in creating classic arcade games. Guide me step-by-step in building a fully functional Snake game using Python and Pygame. Include details on setting up the game window, handling keyboard inputs for snake movement, implementing collision detection, and tracking the score. Also, explain how to add [GAME DIFFICULTY LEVELS], [CUSTOM SNAKE SKINS], and [MULTIPLAYER MODE] as optional enhancements. Provide clean, commented code snippets and best practices for structuring the project.