Elytra Slot

在玩家的物品栏中添加背部槽位，允许玩家将鞘翅放在该槽位中。可以在不牺牲胸甲槽位的情况下使用鞘翅。
Add a back slot to the player's inventory, allowing players to place elytra in this slot. This lets players use elytra without occupying the chestplate slot.