New reference panel: Display a stack of definitions a user jumped along in reference code blob

Created by: mrnugget

See https://github.com/sourcegraph/sourcegraph/issues/2065 and https://sourcegraph.slack.com/archives/C0W2E592M/p1626947337185400 and https://cs.opensource.google/go/go/+/master:src/bytes/buffer.go;l=30;drc=master