Extension API request: ability to know when a user clicked on a token

Created by: chrismwendt

Motivating use case for token-highlights: https://github.com/sourcegraph/sourcegraph-extension-samples/issues/10

In terms of implementation, my first thought was to add a function, registerTokenClickProvider, similar to the hover/def/ref ones. However, unlike the others, there's no action embedded in the words TokenClick. It's more like a fire-and-forget event than a request because the return type would be void.

cc @lguychard