Allow `CodeEditorControl` to be instantiated with a `editor_settings` param which is merged with `wp.codeEditor.defaultSettings`.
Also:
* Turn redundant "CSS Code" control label into screen reader text for Additional CSS.
* Remove `code-editor` as script dependency for `custom-html-widgets` since enqueueing is determined by `wp_enqueue_code_editor()`.
* Remove useless exporting of `code_type` param to JS in `WP_Customize_Code_Editor_Control`.
* Add `disabled` class to Custom HTML widget's Save button when linting errors are present.
* Remove redundant `span` inside CodeEditorControl's `label`.
See #41897, #12423, #41872.
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- 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.
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* 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
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Weston Ruter
Andrea Fercia