Feature request: call hierarchy

[sqs]

For answering:

• "Where is this function called from?"
• "How is this function being used?" when the precise args or context lines matter
• "What tests call this function?" (would require a way to filter calls by "some level in the hierarchy is a function whose name contains test" or similar)

This is best demonstrated by a future VS Code feature with the same name, described at microsoft/vscode#16110 (comment). Screenshot:

(Split off from https://github.com/sourcegraph/sourcegraph/issues/2278, related to #2882 and #2883)

[macraig]

Closing due to inactivity.