Add startpage onion service to list of search providers

Startpage has added a new onion service, so lets add it to the list of built-in search providers.

URL: http://startpagel6srwcjlue4zgq3zevrujfaow726kjytqbbjyrswwmjzcqd.onion/

We'd like this in the next stable release 13.0.12 and the next alpha release 13.5a6