The warning displayed upon first visit on the File Editors introduced in [41774]
needs to be the only perceivable content in the page for users of assistive
technologies. It looks like a modal but it's not exactly an ARIA dialog, not an
ARIA alert either, and needs some special treatment.
- constrains tabbing within the modal
- uses `wp.a11y.speak()` to make screen readers announce the modal message
- hides all the other page content from assistive technologies using `aria-hidden="true"`
This way, even if users miss the speak message, the warning is actually the only
perceivable content in the page.
Fixes#42110.
Built from https://develop.svn.wordpress.org/trunk@41876
git-svn-id: http://core.svn.wordpress.org/trunk@41710 1a063a9b-81f0-0310-95a4-ce76da25c4cd