DataTransferItemList should be a class with a non-public constructor

Co-authored-by: Arman Bilge <armanbilge@gmail.com>

Author: zetashift

dom/src/main/scala/org/scalajs/dom/DataTransferItemList.scala

@@ -14,7 +14,7 @@ import scala.scalajs.js.annotation.JSGlobal
   */
 @js.native
 @JSGlobal
-class DataTransferItemList extends js.Object {
+class DataTransferItemList private[this] extends js.Object {
 
   /** Returns the number of items in the drag data store. */
   def length: Int = js.native