set_damage item modifier to mirror enchantment effect change_item_damage

since enchantments are going to have damage_item changed to change_item_damage, with support for negative values and already supporting whole numbers that don't change based on total durability. item modifiers only having the option to change by a fraction makes having items with different total durabilities require different item modifiers in order to change their durabilities by whole numbers, and with the enchantment effect changes i think it only makes sense to also change what item modifiers can do regarding durability