Add syntax highlighting to code editor

[karmatosed]

This has come up a few times in testing. When you are in the code editor or a block that shows code, having syntax highlighting is something people are asking for. My question here is would there be a light weight highlighter we could use that doesn't harm performance?

[afercia]

@karmatosed there's already #1733 open for this, with some good pros and cons argumentations already there 🙂

[westonruter]

For adding CodeMirror to the Custom HTML block in particular, see #1625 (comment)

[karmatosed]

Closing as we have #1733