Add extension API for hover alerts
Created by: shrouxm
summary of changes:
- expose a new 'alerts' field on hovers in sourcegraph extensions, which allows extensions to use the existing alert functionality to display non-content messages on hovers
- render alerts using markdown instead of react fragments so they can be passed through sourcegraph extensions
- make alerts dismissible on dot-com as well as in browser extension
- allow badges on alerts
closes #10925 along with an upcoming code-intel-extensions PR. #10926 and #10927 require some additional backend changes which might slip.
@sourcegraph/web sorry for the late submission, but I really want to merge this before branch cut so a review today would be much appreciated! I will be adding tests later today.