LibWeb: Implement the DataTransferItem kind and type attributes

Author: https://github.com/trflynn89
Commit: 001d8384e5
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1158
Reviewed-by: https://github.com/shannonbooth

Tests/LibWeb/Text/expected/HTML/data-transfer.txt

@@ -1,7 +1,7 @@
 dropEffect: none
 effectAllowed: none
 length=0, types=
-stringItem: [object DataTransferItem]
+stringItem: kind=string, type=custom-type
 length=1, types=custom-type
-fileItem: [object DataTransferItem]
+fileItem: kind=file, type=text/plain
 length=2, types=custom-type,Files