[UI] Add CodeMirror to Hedy

[jpelay]

Description

Adds the most basic support for CodeMirror in Hedy!

Note: I've included the changes to the interface that I did in #4435 and will keep this in sync with that PR. Just wanted to get this going as soon as possible.

Fixes #4461

How to test

• If this is a UI change: describe how to run your code to see it in action. See this Stop long running programs in client-side JS #880 (comment) for an example
• If this is a language change: add a few tests showing the difference

Checklist
Done? Check if you have it all in place using this list: (mark with x if done)

• Contains one of the PR categories in the name
• Describes changes in the format above
• Links to an existing issue or discussion
• Has a "How to test" section

If you're unsure about any of these, don't hesitate to ask. We're here to help!