Make item_draw_specification availible to read at runtime

[Osmo]

Add https://lua-api.factorio.com/latest/pro ... cification to the runtime LuaEntityPrototype. Useful for mods that want to draw their own icons over entites such as https://mods.factorio.com/mod/alt-alt-mode.

[LeonSkills]

As the author of that mod, support.

Same for EntityPrototype.icons_positioning.

For now I'm gathering the info in a lua file to be read at runtime, but preferably I'd just use LuaEntity.prototype.item_draw_specification so it also works for all modded entities.