Fullscreen Mode and Page for Smaller Screens

Hi! Horse-o-chess' window was a bit large for some smaller screens so I added a couple things that should hopefully help.
-You can now press F to toggle full-screen

-You can play the game at this link in a smaller window: https://pajamabeevegan.itch.io/horse-o-chess-small-screen

The full-screen might not work for everyone, but hopefully it's useful for some :)