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.

Built from https://develop.svn.wordpress.org/trunk@41586


git-svn-id: http://core.svn.wordpress.org/trunk@41419 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-controls.css

@@ -550,12 +550,15 @@ p.customize-section-description {
 	margin-top: 22px;
 	margin-bottom: 0;
 }
+
 .customize-section-description ul {
 	margin-left: 1em;
 }
+
 .customize-section-description ul > li {
 	list-style: disc;
 }
+
 .section-description-buttons {
 	text-align: right;
 }
@@ -1197,12 +1200,17 @@ p.customize-section-description {
 	height: 500px;
 }
 
+.customize-section-description-container + #customize-control-custom_css .customize-control-title {
+	margin-left: 12px;
+}
+
 .customize-section-description-container + #customize-control-custom_css:last-child textarea {
 	border-right: 0;
 	border-left: 0;
 	height: calc( 100vh - 185px );
 	resize: none;
 }
+
 .customize-section-description-container + #customize-control-custom_css:last-child {
 	margin-left: -12px;
 	width: 299px;
@@ -1213,6 +1221,7 @@ p.customize-section-description {
 .customize-section-description-container + #customize-control-custom_css:last-child .CodeMirror {
 	height: calc( 100vh - 185px );
 }
+
 .CodeMirror-lint-tooltip,
 .CodeMirror-hints {
 	z-index: 500000 !important;
@@ -1227,6 +1236,7 @@ p.customize-section-description {
 	.customize-section-description-container + #customize-control-custom_css:last-child {
 		margin-right: 0;
 	}
+
 	.customize-section-description-container + #customize-control-custom_css:last-child textarea {
 		height: calc( 100vh - 140px );
 	}