[Bleeding] Add iterator() with starting index. Addresses BUKKIT-1246

By: Celtic Minstrel <celtic.minstrel.ca@some.place>

paper-api/src/main/java/org/bukkit/inventory/Inventory.java

@@ -272,4 +272,13 @@ public interface Inventory extends Iterable<ItemStack> {
     public InventoryHolder getHolder();
 
     public ListIterator<ItemStack> iterator();
+
+    /**
+     * Returns an iterator starting at the given index. If the index is positive, then the first
+     * call to next() will return the item at that index; if it is negative, the first call to
+     * previous will return the item at index (getSize() + index).
+     * @param index The index.
+     * @return An iterator.
+     */
+    public ListIterator<ItemStack> iterator(int index);
 }