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/DragDataStore.h

@@ -48,6 +48,7 @@ public:
 
     void add_item(DragDataStoreItem item) { m_item_list.append(move(item)); }
     ReadonlySpan<DragDataStoreItem> item_list() const { return m_item_list; }
+    size_t size() const { return m_item_list.size(); }
     bool has_text_item() const;
 
     Mode mode() const { return m_mode; }