9bec434d moved the logic for scrolling
to an anchor into the document integrations, in order to enable
custom logic depending on the document type. The bucket bar however
was still using a generic approach.

Change the bucket bar to use the same `scrollToAnchor` implementation as
when scrolling is triggered by clicking an annotation card in the
sidebar.

 - Add `scrollToAnchor` method to `Guest`
 - Modify bucket bar controller to use this method

Related to https://github.com/hypothesis/client/issues/3269