Simplify usage by removing scene loaders

[joaoborks]

Since version 3.0.0, the scene loaders have been very stripped down in functionality. The transition method can easily be migrated to the scene manager, and the entire usage can be simplified by removing the scene loaders.

-scene loader > advanced scene manager > unity scene manager
+advanced scene manager > unity scene manager

Additionally, this change would provide the opportunity to replicate the current Unity SceneManager usage by having a static implementation available. It can be hooked to the runtime initialization via RuntimeInitializeOnLoadMethod attribute to build a global scoped scene manager.