Add registry for Entities

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.

Start by tracing Entity.fromJson() and SceneEntities.loadEntitiesInChunk(), then inspect how the Scene tab builds the “Load Entities” dropdown. Define registration and lookup behavior for class and string IDs, including plugin entities; done means all three paths can resolve registered entities.

Written by the indexing model from the issue text.