codeintel: Token-click jump to definition

Created by: chrismwendt

This makes clicking on a token jump to its definition.

https://user-images.githubusercontent.com/1387653/144978118-af423845-c201-428d-bc92-07a1feb36fe9.mp4

It doesn't interfere with selecting text:

Try it out:

• git checkout token-click-j2d
• sg start enterprise-codeintel
• Open https://sourcegraph.test:3443/github.com/sourcegraph/src-cli/-/blob/internal/batches/docker/cache.go
• Click around 🎉

TODO

• Do the same for references when you're already on the definition
• Record telemetry to track token-clicks for defs/refs vs hover popover

Resolves https://github.com/sourcegraph/sourcegraph/issues/28036