Announce server messages to screen readers; hide them with the hidden attribute

Problem/Motivation

Server messages are rendered as hidden elements. js/izi_message.js shows each one as an iziToast toast and removes the element. This has two problems:

• Screen readers do not hear these messages. iziToast adds no role or aria-live, and the hidden element is removed. JS messages (Drupal.Message) are announced, because core's add() calls Drupal.announce().
• The wrapper is hidden with an inline style="display: none !important;". A Content Security Policy without 'unsafe-inline' for styles blocks it, so the messages show as plain text until the toasts appear.

Proposed resolution

• js/izi_message.js: call Drupal.announce() with the title and the text of each message. Use 'assertive' for errors, as core does. Add core/drupal.announce to the izi_message/izi_message library.
• templates/izi-message.html.twig: replace the inline style with the hidden attribute. The fallback without the iziToast library removes the attribute.
• README and help page: note that without JavaScript the server messages are not shown (known limitation).

Remaining tasks

• FunctionalJavascript test: a server message is in the #drupal-live-announce region.
• Check that a theme override of izi-message.html.twig with the inline style still works.
• MR, review.

User interface changes

Screen readers announce server messages.