JavaScript public event for customizing codemirror instances

[ssddanbrown]

Similar to existing event:
https://github.com/BookStackApp/BookStack/blob/development/dev/docs/javascript-public-events.md#library-cm6configure-theme

Right now, there's no way to access/customize CM instances since it's fairly deep in the code.
Would be nice to provide a point for high-level hackery.
Would also need to expose some CM objects.
Challenge may be in how it's visible whether an instance is the MD editor, settings option or a view-only block. Need to provide this context.

Enable code wrapping as an example: #3272