Add CartItem class for order item tracking

src/main/java/dev/model/CartItem.java

@@ -1,4 +1,23 @@
 package dev.model;
 
 public class CartItem {
+    private final MenuItem item;
+    private final int quantity;
+
+    public CartItem(MenuItem item, int quantity) {
+        this.item = item;
+        this.quantity = quantity;
+    }
+
+    public MenuItem getItem() {
+        return item;
+    }
+
+    public int getQuantity() {
+        return quantity;
+    }
+
+    public double getSubtotal() {
+        return item.getPrice() * quantity;
+    }
 }