Add support for call hierarchy

[DonJayamanne]

See microsoft/language-server-protocol#468

The preview feature has landed in VS Code
Not sure about support in LSP, but support is available for extensions.
https://code.visualstudio.com/updates/v1_33#_call-hierarchy

[GuSuku]

@DonJayamanne You had closed the issue in Python language extension, Call Hierarchy Implementation microsoft/vscode-python#5127, referring to this current issue as:

Duplicate of upstream issue microsoft/python-language-server#888

But in this current issue above you state:

Not sure about support in LSP, but support is available for extensions.

Sounds inconsistent, unless I am missing something.

[erictraut]

Call Hierarchy support is implemented in Pylance, the Microsoft's next-generation Python Language Server. Please give it a try and report any new issues here.