* Adds `WP_Customize_Code_Editor_Control` and `wp.customize.CodeEditorControl()`.
* Control respects user preference for syntax highlighting, showing a textarea when user opts out.
* Code editor control takes the ad hoc code for Additional CSS and makes it reusable and extensible, for Additional CSS in core and plugins to use (such as Jetpack).
* Replace `settings` arg in `wp_enqueue_code_editor()` with separate args for `codemirror`, `csslint`, `jshint`, and `htmlhint`.
* Prefix `codemirror` script and style handles with `wp-` to prevent collisions, as also the object is exported as `wp.CodeMirror` in JS.
* Reduce indent size in Customizer code editor instances and Custom HTML widget to use tab size of 2 instead of 4 to save on space.
See #12423, #38707, #35395.
Fixes#41897.
git-svn-id: https://develop.svn.wordpress.org/trunk@41558 602fd350-edb4-49c9-b593-d223f7449a82