Alpha version

This is the first version, so there are a few flaws:

- The movement has no decay, so when you move, you won't slow down over time or even stop. Due to this, I made it so that when moving backward (DOWN/W key) you will move faster to create a kind of "brake" mechanic.

- While the asteroids spawn near the border of the window, they still appear out of thin air, which may collide with the ship and costing a life.

- The asteroids phase through each other, so there is not much physics yet.

- Bullets one-shot the asteroids and asteroids one-shot the ship. There is no HP mechanic yet.