LuaEntityPrototype.inventory_size

[Mooncat]

Request:
Add the inventory_size property for LuaEntityPrototype so we know the inventory size of chest prototypes without the need of getting any chest on the map.

[Rseding91]

Added for 0.15: get_inventory_size on LuaEntityPrototype that uses the same inventory indexes as get_inventory.

[Mooncat]

Thanks! <3