Added demo

Add a demo in the form of a game. Assets from the following authors were used:

Dummy sprite and Hit effect - https://otsoga.itch.io/training-dummy
Tileset - https://xzany.itch.io/fantasy-tileset-2d-pixel-art
Music - https://alkakrab.itch.io/free-12-tracks-pixel-rpg-game-music-pack
Sound effects - https://robingather.itch.io/any-journey-rpg-pack