Extension contributed hover tooltips precede default tooltips

Created by: nicksnyder

Expected: I expect the default code intelligence documentation in hover tooltips to always come first, no matter what extensions I install.

Actual:

Reproduce by following the instructions here: https://github.com/mhewett/sourcegraph-coding-examples/issues/2#issue-375222892

One way to detect is "does the extension also provide a definition and/or reference provider"? If so, it should be the first hover tooltip.

cc @chrismwendt @sqs @felixfbecker @ijsnow @slimsag