Add support for CKEditor 5

Problem/Motivation

After upgrading from CKEditor 4 to CKEditor 5, the warning popup no longer appears if I edit the text in a CKEditor 5 box and then try to leave the page without saving.

Proposed resolution

The condition in js_confirm_pop_up.js currently works with CKEditor 4:

            // Handle backbutton, exit etc.
            window.onbeforeunload = function () {
                // Add CKEditor support.
                if (typeof (CKEDITOR) != 'undefined' && typeof (CKEDITOR.instances) != 'undefined') {
                    for (var i in CKEDITOR.instances) {
                        if (CKEDITOR.instances[i].checkDirty()) {
                            edit = true;
                            break;
                        }
                    }
                }

Add a second condition that works with CKEditor 5:

            // Handle backbutton, exit etc.
            window.onbeforeunload = function () {
                // Add CKEditor support.
                if (typeof (CKEDITOR) != 'undefined' && typeof (CKEDITOR.instances) != 'undefined') {
                    for (var i in CKEDITOR.instances) {
                        if (CKEDITOR.instances[i].checkDirty()) {
                            edit = true;
                            break;
                        }
                    }
                }
                
                // Add CKEditor 5 support, using technique from               
                // https://www.drupal.org/project/node_edit_protection/issues/3364707#comment-15453890
                if (document.querySelector('[data-ckeditor5-id][data-editor-value-is-changed=true]')) {
                  edit = true;
                }