- 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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