Embedded Language Support + LiveDoc

[zampino]

• Make clojure-mode embeddable into markdown
• Experiment with editable/preview markdown blocks
• Depends on https://github.com/nextjournal/clerk/pull/195

[zampino]

I think we could merge this now, we will address the following remaining issues/todos in separate steps.

• cannot click to move cursor in editable sections bottom lines (probably we need calls to requestMeasure)
• scroll selected block into view when moving out of viewport
• clicking on blocks not always results in an edit at the right place or at all
• avoid re-rendering all previews when scrolling or clicking to edit one (probably connected to height computations)
• use async SCI eval
• don't eval code when rendering previews (but only when leaving edit mode)