MDL-75078 editor_tiny: Hola TinyMCE 6

Part of MDL-75966

This commit adds TinyMCE version 6.1 into Moodle core.

A basic editor implementation is included, along with Moodle loader, and
basic Plugin API for Moodle. The Plugin API will be extended and
improved in future changes.

A new subplugin is defined for this plugin.

lib/classes/plugin_manager.php

@@ -1861,7 +1861,7 @@ class core_plugin_manager {
             ),
 
             'editor' => array(
-                'atto', 'textarea', 'tinymce'
+                'atto', 'textarea', 'tiny', 'tinymce'
             ),
 
             'enrol' => array(
@@ -2028,6 +2028,9 @@ class core_plugin_manager {
                 'objectives'
             ),
 
+            'tiny' => [
+            ],
+
             'tinymce' => array(
                 'ctrlhelp', 'managefiles', 'moodleemoticon', 'moodleimage',
                 'moodlemedia', 'moodlenolink', 'pdw', 'spellchecker', 'wrap'