Add missing rarity method on ItemStack (#5594)

Spigot-API-Patches/Item-Rarity-API.patch

@@ -86,3 +86,23 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +    public io.papermc.paper.inventory.ItemRarity getItemStackRarity(ItemStack itemStack);
      // Paper end
  }
+diff --git a/src/main/java/org/bukkit/inventory/ItemStack.java b/src/main/java/org/bukkit/inventory/ItemStack.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/main/java/org/bukkit/inventory/ItemStack.java
++++ b/src/main/java/org/bukkit/inventory/ItemStack.java
+@@ -0,0 +0,0 @@ public class ItemStack implements Cloneable, ConfigurationSerializable, net.kyor
+         ItemMeta itemMeta = getItemMeta();
+         return itemMeta != null && itemMeta.hasItemFlag(flag);
+     }
++
++    /**
++     * Gets the item rarity of the itemstack. The rarity can change based on enchantements.
++     *
++     * @return the itemstack rarity
++     */
++    @NotNull
++    public io.papermc.paper.inventory.ItemRarity getRarity() {
++        return Bukkit.getUnsafe().getItemStackRarity(this);
++    }
+     // Paper end
+ }