Colliding with other sprites

Let's fix the issue with George wandering off the edge of the screen that we saw at the end of the last section and flip the direction if he hits the edge of the screen. To do this, follow these steps:

1. In the onLoad function of main.dart, let's add the ScreenCollidable component to the bottom of the function so we can detect collisions between George and the screen edges:

   add(ScreenCollidable());

2. In the george.dart file, change the class definition to add the HasHitBoxes and Collidable mixins:

   class George extends SpriteAnimationComponent with HasHitBoxes, Collidable {

3. At the bottom of the onLoad function, add the HitboxRectangle shape for the collision detection:

   addHitbox(HitboxRectangle());

4. Add the following import at the top of the file to resolve the reference to HitboxRectangle:

   import 'package:flame/geometry.dart';

5. At the bottom of this class file, after the update function definition, add the following onCollision...