Make LuaEntity.previous_recipe writeable

[Thremtopod]

As I understand it, LuaEntity.previous_recipe is only used for setting the icon of a no-longer-crafting furnace, and it has no effect on any actual behavior of the entity. It would be a nice QoL feature for my mod (which can "hot-swap" a furnace with an assembling-machine version of it, and back) to be able to write to this field, so that when an assembling machine is "swapped" to its furnace version, it can retain its recipe icon even when it's out of ingredients. I don't need the field to actually do anything else; it would just be better UX for the player for the icons to be consistent.