LibWeb: Implement the DataTransferItemList indexed getter

Author: https://github.com/trflynn89
Commit: ceb9e30d42
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1158
Reviewed-by: https://github.com/shannonbooth

Userland/Libraries/LibWeb/HTML/DataTransfer.h

@@ -61,6 +61,7 @@ public:
 
     JS::NonnullGCPtr<DataTransferItem> add_item(DragDataStoreItem item);
     bool contains_item_with_type(DragDataStoreItem::Kind, String const& type) const;
+    JS::NonnullGCPtr<DataTransferItem> item(size_t index) const;
     size_t length() const;
 
 private: