Customize: Introduce extensible code editor Customizer control for CodeMirror.

* 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.

Built from https://develop.svn.wordpress.org/trunk@41558


git-svn-id: http://core.svn.wordpress.org/trunk@41391 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/widgets/class-wp-widget-custom-html.php

@@ -160,6 +160,10 @@ class WP_Widget_Custom_HTML extends WP_Widget {
 	public function enqueue_admin_scripts() {
 		$settings = wp_enqueue_code_editor( array(
 			'type' => 'text/html',
+			'codemirror' => array(
+				'indentUnit' => 2,
+				'tabSize' => 2,
+			),
 		) );
 
 		wp_enqueue_script( 'custom-html-widgets' );