SPIGOT-6890: Add repair cost amount in AnvilInventory

By: Doc <nachito94@msn.com>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryAnvil.java

@@ -27,6 +27,16 @@ public class CraftInventoryAnvil extends CraftResultInventory implements AnvilIn
         return container.itemName;
     }
 
+    @Override
+    public int getRepairCostAmount() {
+        return container.repairItemCountCost;
+    }
+
+    @Override
+    public void setRepairCostAmount(int amount) {
+        container.repairItemCountCost = amount;
+    }
+
     @Override
     public int getRepairCost() {
         return container.cost.get();