Fix CraftStructureTransformer from deleting generated entities

[Machine-Maker]

CraftLimitedRegion loads and stores entities in the protochunk and doesn't parse the entities from NBT each time. This requires that a new instance of CraftLimitedRegion is created each time before passing it to an api consumer and then saving/clearing it afterwards.

CraftStructureTransformer was creating a single instance for an entire structure place which was causing entities to be deleted so no structures would spawn entities.

Demo to cause the issue. Just add this listener and teleport around to newly generated villages.

@EventHandler
public void onEvent(AsyncStructureGenerateEvent event) {
    event.setEntityTransformer(new NamespacedKey(this, "test"), (region, x, y, z, entity, allowedToSpawn) -> {
        System.out.println(region.getEntities());
        return true;
    });
}

[Machine-Maker]

Alternative fix would be to add the entity to the list stored in CraftLimitedRegion if the entities were loaded. That way you can re-use the instances and they will stay up-to-date.