Optionally support codemirror_editor for yaml editing (if installed)

Added codemirror_editor in the require section of composer.json with version 1 and 2 ~~ not sure is there any configuration required.

@anybody: quick question why are we supporting D8 for this module ~~ any specific reason?

Sorry, this should not go into composer, as it's just an optional dependency ("suggests" in composer if you'd like to).

This needs to be implemented where the textarea is displayed and if module is enabled, codemirror_editor should be used, like yaml_editor already (which could also go into composer suggests)