Type hierarchy provider

[lars-reimann]

LSP 3.17 added a way to get the subtypes/supertypes of an entity, based on three requests:

• textDocument/prepareTypeHierarchy
• typeHierarchy/supertypes
• typeHierarchy/subtypes

It would be nice to be able to easily hook into this, similar to the CallHierarchyProvider.

I'm currently working on this for a Langium-based DSL:

• TypeHierarchyProvider
• AbstractTypeHierarchyProvider
• request handler

The interface is basically a copy of the CallHierarchyProvider. If you'd accept such a PR, I'd like to move the interface, the no-op implementation, and the request handler up into Langium.

[msujew]

Sure, that sounds pretty good. We'd love a contribution in that direction 👍