This was static to permit ItemFactory to register spawn eggs for all known entity types in early PM4. However, nowadays we provide a callback to the spawn egg instead, and spawn eggs must be manually implemented, so this is no longer needed.
In addition, having this static forces everyone to make a new entity class for every unique type of entity, which isn't ideal.
making this nullable was based on the invalid assumption that global sounds have no position, but it turns out they _do_ still use the position to make the sound come from the correct direction.
