Return the item entities from LuaSurface::spill_item_stack

[Mooncat]

Request:
Returns the created item-entities from LuaSurface::spill_item_stack, so we can immediately mark them for deconstruction or do other stuff on them.

Alternatively, allow the "spill" parameter in LuaSurface::create_entity to simulate item spilling when "item-on-ground" is created, regardless of "fast_replace".