Add registry for Entities

[ThisIsBrady]

Entities should be registerable given an Entity class and a string ID or set of IDs. This would allow the UI to automatically add the entity to the "Load Entities" dropdown in the Scene tab, the Entity class to automatically be identified from JSON (Entity.fromJson()) or NBT tag (SceneEntities.loadEntitiesInChunk()), and make it feasible for plugins to add their own entities.

[leMaik]

Good to have an issue for this. Once we add this, we should probably create an example plugin that adds one entity.