LibWeb: Implement DataTransferItem.prototype.getAsString()

Author: https://github.com/trflynn89
Commit: 181ece4d9c
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1163
Reviewed-by: https://github.com/tcl3 ✅

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

@@ -3,6 +3,7 @@ effectAllowed: none
 length=0, types=
 stringItem: kind=string, type=custom-type
 length=1, types=custom-type
+stringItemAsString: data=well hello friends
 fileItem: kind=file, type=text/plain
 length=2, types=custom-type,Files
 fileItemAsFile: name=file.txt, type=text/plain