update: menu, new puzzle, and some other things

Changes:

• Added a main menu and a level selection screen.
• Added a new level 4 to demonstrate the fact that you can loop around nothing.
• Made level 7 (formerly level 6) slightly easier by removing one of the goal spaces.
• Added hints.
• Added WASD to move.
• Added a sound effect when you try to move in the opposite direction to where you’re facing.

Thanks to Niel McLaren (https://nielmclaren.itch.io/) for giving very helpful feedback!