feat: allow extensions to display d3 charts with pure markdown

Created by: dadlerj

Experimental feature, not necessary if there are any risks here. I'd like to be able to contribute basic charts from extensions (e.g. git data, usage data for admins, etc.).

Enables things like this: https://sourcegraph.com/extensions/dadlerj/sourcegraph-contributors-extension

Uses https://sourcegraph.com/github.com/geekplux/markvis-bar, and https://sourcegraph.com/github.com/geekplux/markvis-line