LibWeb: Add an empty DataTransfer IDL implementation

This does not implement any of the IDL methods, but GitHub requires the
interface exists to upload files via an <input type="file"> element.
Their JS handles uploads via this element and via drag-and-drop in one
function, and check if the uploaded file is `instanceof DataTransfer` to
decide how to handle it.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -260,6 +260,7 @@ set(SOURCES
     HTML/DocumentState.cpp
     HTML/DOMParser.cpp
     HTML/DOMStringMap.cpp
+    HTML/DataTransfer.cpp
     HTML/ErrorEvent.cpp
     HTML/EventHandler.cpp
     HTML/EventLoop/EventLoop.cpp