(Split from #15485)

Dart Code has had requests for viewing/navigating type hierarchies. We've implemented this for now using the pick-list, but we had to make compromises to fit it into a flat list.

Some other editors have this functionality and it's a concept that's not specific to Dart, so it might make sense as a provider that can be implemented by extensions.

Here's an image of it in another editor and also how we've managed to implement it currently.