Add powerup scene

[davcri]

Add a powerup scene.

We need to:

• Create a Sprite
• Create a Godot scene with
  • Area2D with shape (for collision detection)
• Add a little description of the powerup in the GDD

You can take inspiration from here: https://github.com/crystal-bit/space-shooter/blob/master/game-design-doc/game-design-document.md#fire-bullet---powerup1

We don't need to implement the spawn in this issue.