LibWeb: Implement the DataTransferItemList length attribute

Author: https://github.com/trflynn89
Commit: 843f8f04a5
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;
+    size_t length() const;
 
 private:
     DataTransfer(JS::Realm&, NonnullRefPtr<DragDataStore>);