Add ignoreScrollBehavior to new API

This PR exposes `ImitateBrowserBehavior` and `ScrollToTopBehavior` and reintroduces `ignoreScrollBehavior` which allows you to opt out of scrolling changes within bounds of some ancestor route.

This was already implemented in #388 but it is missing in new API so I'm reimplementing it.
This PR adds test for `ScrollToTopBehavior` but lacks tests for `ignoreScrollBehavior`. I'll add these shortly.