MDL-80257 editory_tiny: Implement ui_mode: split

Since version 6.4 and later, TinyMCE had a feature `ui_mode: split`.

It enables support for editors in scrollable containers,
and Moodle has some pages, especially the course pages, that are using the scrollable containers.
Therefore, all workaround that is addressed should be replaced by the feature to avoid future problems
regarding the overflow and the z-index issues.

lib/editor/tiny/classes/editor.php

@@ -194,9 +194,6 @@ class editor extends \texteditor {
 
             // Plugin configuration.
             'plugins' => $this->manager->get_plugin_configuration($context, $options, $fpoptions, $this),
-
-            // Nest menu inside parent DOM.
-            'nestedmenu' => true,
         ];
 
         if (defined('BEHAT_SITE_RUNNING') && BEHAT_SITE_RUNNING) {