Small optimizations to multiple scripts

- New abstract class Collectable is extensible for all items
- SilverBell class extends Collectable
- Unnecessary EnemyDiving states were removed
- PlayEndSequence Coroutine added to SceneManager
Add new collectables and fix EnemyDiving

- Create new prefabs for Silver Bell and Speed up power-up
- Fix the EnemyDiving path to intersect player again