Create a "TypeHierarchyProvider" api for visualising type hierarchies

[DanTup]

(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.

[fwcd]

Any progress on this one? I would really love to have this feature in VSCode.

[KamasamaK]

@fwcd Not for VS Code native yet, but there has been recent activity on the LSP implementation at microsoft/vscode-languageserver-node#426, which when completed generally translates into a very similar native API. Of course, there's also a matter of the UI.

[tomhanax]

We have call hierarchy now, which is perfect. How it is going with type hierarchy? This is IMHO somewhat essential functionality in typed languages like TypeScript, Java...

[akaroml]

+1 from Java perspective. Type Hierarchy is also super useful for Java users.