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

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

@@ -1,4 +1,7 @@
 dropEffect: none
 effectAllowed: none
-stringItem: [object DataTransferItem], types=custom-type
-fileItem: [object DataTransferItem], types=custom-type,Files
+length=0, types=
+stringItem: [object DataTransferItem]
+length=1, types=custom-type
+fileItem: [object DataTransferItem]
+length=2, types=custom-type,Files