Accessibility: CodeMirror editing areas minor improvements.

- 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.


git-svn-id: https://develop.svn.wordpress.org/trunk@41586 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/customize/class-wp-customize-code-editor-control.php

@@ -71,6 +71,7 @@ class WP_Customize_Code_Editor_Control extends WP_Customize_Control {
 		$json = parent::json();
 		$json['code_type'] = $this->code_type;
 		$json['editor_settings'] = $this->editor_settings;
+		$json['input_attrs'] = $this->input_attrs;
 		return $json;
 	}
 
@@ -98,7 +99,11 @@ class WP_Customize_Code_Editor_Control extends WP_Customize_Control {
 			<span class="description customize-control-description">{{{ data.description }}}</span>
 		<# } #>
 		<div class="customize-control-notifications-container"></div>
-		<textarea id="{{ elementIdPrefix }}_editor" class="code"></textarea>
+		<textarea id="{{ elementIdPrefix }}_editor"
+			<# _.each( _.extend( { 'class': 'code' }, data.input_attrs ), function( value, key ) { #>
+				{{{ key }}}="{{ value }}"
+			<# }); #>
+			></textarea>
 		<?php
 	}
 }