Adding New Power-ups

Power-ups are items that spawn on the map and provide bonuses to tanks.

1. Create the Bonus Class: Create a new file in src/entities/powerups/.
2. Inherit from PowerUp: Use the PowerUp base class.
3. Define Effect: Implement what happens when a tank collects the item.
4. Register: Add to src/entities/powerups/index.ts so it can be spawned by the Game instance.