Consolidate CheckpointManager into SceneManager

- Make adjustments to camera logic
- Tweak player physics values
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
Add proper deceleration to actor and player
Add Sliding functionality to actor
Add sliding animation, tweak player movement
Fix issues with sliding on sloped terrain

Also:
- Separate death volume logic into its own class
- Implement new player collision logic
*NOTE: Remove this new collision logic from SilverBell.cs
Fix sliding and stopping under death volume issue
Fix bug where player gets hung up on hills
Clean up a bunch of unneeded stuff!

- remove any unused plugins
- remove old 2D assets
- Final adjustments before new level creation
I lied! Clean up more stuff!

- Parent the light to the camera
- Create a couple new prefabs
Build some of level 1, prep camera

- Blocked out some code in CameraMan to follow the player vertical if
she gets too close to the upper and lower border.