CKEditor 5 changes are not tracked by "unsaved changes" warning

Problem/Motivation

webform.form.unsaved.js tracks unsaved changes on forms carrying the js-webform-unsaved class by binding change/keypress on inputs, plus a dedicated block for CKEditor:

  // Track all CKEditor change events.
  // @see https://ckeditor.com/old/forums/Support/CKEditor-jQuery-change-event
  if (window.CKEDITOR && !CKEDITOR.webformUnsaved) {
    CKEDITOR.webformUnsaved = true;
    CKEDITOR.on('instanceCreated', function (event) {
      event.editor.on('change', function (evt) {
        unsaved = true;
      });
    });
  }

This was added in #2989090 (2018) when CKEditor 4 was the only editor available in Drupal core. CKEditor 5 has no window.CKEDITOR global, so this block never runs, and CKEditor 5 doesn't fire native change/keypress on its source <textarea> either (it only syncs the textarea value lazily on save/detach). The result: editing a CKEditor 5 field on a js-webform-unsaved form does not set the unsaved flag, so navigating away shows no warning, while editing a plain text field does.

Steps to reproduce

1. Go to /admin/structure/webform/config.
2. Enable Warn users about unsaved changes for all webforms and save.
3. On any form, add an element that uses CKEditor, e.g. Text format with Full HTML.
4. Open the form and make a change to the CKEditor field and navigate to another page -> no This page is asking you to confirm that you want to leave... alert shown.
5. Editing other elements and then navigating away will show the alert.

Proposed resolution

Drupal core's own editor integration already fires a formUpdated event on the field when CKEditor 5 content changes (core/modules/editor/js/editor.js, via Drupal.editors[format].onChange(...)). webform.form.unsaved.js could probably listen for that too, instead of/in addition to the legacy window.CKEDITOR check.