Implement textDocument/callHierarchy

[rwols]

Is your feature request related to a problem? Please describe. I want to see an overview of the callers and callees of a callable.

Describe the solution you'd like Implement textDocument/callHierarchy in the upcoming 3.16 spec.

Describe alternatives you've considered textDocument/references

Additional context clangd can do this so it would be nice to have this.

[rwols]

Relevant requests and links:

textDocument/prepareCallHierarchy https://github.com/microsoft/language-server-protocol/blob/gh-pages/_specifications/specification-3-16.md#textDocument_prepareCallHierarchy

callHierarchy/incomingCalls https://github.com/microsoft/language-server-protocol/blob/gh-pages/_specifications/specification-3-16.md#callHierarchy_incomingCalls

callHierarchy/outgoingCalls https://github.com/microsoft/language-server-protocol/blob/gh-pages/_specifications/specification-3-16.md#callHierarchy_outgoingCalls