Feature request: call hierarchy `textDocument/calls` in hh_server

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](https://github.com/Microsoft/language-server-protocol/issues/468) and [Microsoft/vscode-languageserver-node](https://github.com/Microsoft/vscode-languageserver-node/pull/420).

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