feature: allow drilling down in call hierarchies

[folke]

          Hi @folke, I tried it out, and it's not exactly what I expected from a call-hierarchy UI.

In such UI I expect the incoming calls entries to recursively have their parents in the tree as well. See: https://github.com/marcomayer/calltree.nvim.
This way you could easily see the "call stack" of a function you're inspecting, and quickly jump to any of its ancestors.

I thought that since trouble is a tree UI, it'd be a good fit for such use-case. What do you think?

Originally posted by @shahamran in #222 (comment)