Consider replacing custom `scrolltorange` event with `beforematch`

When the Hypothesis client scrolls to an annotation, a custom `scrolltorange` event is emitted. This event enables the host page to reveal content which might be hidden behind a collapsible section or switch a tab control to the tab containing the content. Chrome recently added a "scroll to text" feature with an associated [`beforematch` event](https://www.chromestatus.com/feature/5400510406328320) which serves a similar purpose. We might want to consider replacing our custom event by emitting `beforematch` in future on the basis that it will be more widely supported by websites.