Make ItemStack final in PreShopCreationEvent class

The change includes making `itemStack` final in the `PreShopCreationEvent` class. It ensures that the item details remain constant and secure during the process of shop creation. This improves data integrity and allows consistent information to be shared with the player during sign creation.

src/main/java/com/Acrobot/ChestShop/Events/PreShopCreationEvent.java

@@ -19,7 +19,7 @@ public class PreShopCreationEvent extends Event {
     private CreationOutcome outcome = CreationOutcome.SHOP_CREATED_SUCCESSFULLY;
     private Sign sign;
     private String[] signLines;
-    private ItemStack itemStack;
+    private final ItemStack itemStack;
 
     public PreShopCreationEvent(Player creator, Sign sign, String[] signLines, ItemStack itemStack) {
         this.creator = creator;