Feature request: call hierarchy textDocument/calls in hh_server

[aosq]

It'd be nice to provide the call hierarchy for a given function (both callees and callers) from hh_server to be ready when this is available in vscode. Looks like some folks are doing some of this work over in the Microsoft/language-server-protocol and Microsoft/vscode-languageserver-node.

This feature is super useful in IntelliJ. It makes exploring code and debugging so much faster :).

[dlreeves]

cc. @ljw1004

[fredemmott]

marking this as external for now, as we're likely to want to have this standardized (or at least have solid progress there) before we do this. @ljw1004 can confirm/remove.