- properly labels all the code editor areas (Theme/Plugin, Custom HTML widget, Additional CSS), whether CodeMirror is enabled or disabled
- adds `role="textbox"` and `aria-multiline="true"` to the CodeMirror editing area to allow assistive technologies properly identify it as a textarea
- standardizes the "keyboard trap" help text across the admin and keeps it as a list for better readability
- use the Help text elements as target for `aria-describedby`, to make screen readers read out the help text when focusing the editors
- fixes the `aria-expanded` attribute usage in the Customizer "Additional CSS" help toggle
- moves focus to the CodeMirror editing area when clicking on the associated label
- in the Plugin editor screen: changes a `<big>` element to `<h2>` for better semantics and consistency with the Theme editor screen
- also, removes a few textdomain leftovers, see `better-code-editing` and `default`
Props westonruter, melchoyce, afercia.
Fixes#41872.
Built from https://develop.svn.wordpress.org/trunk@41586
git-svn-id: http://core.svn.wordpress.org/trunk@41419 1a063a9b-81f0-0310-95a4-ce76da25c4cd
* 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
