MDL-78159 lib: Added missing properties for core libraries

In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.

lib/outputlib.php

@@ -689,6 +689,31 @@ class theme_config {
      */
     public $activityheaderconfig = [];
 
+    /**
+     * For backward compatibility with old themes.
+     * BLOCK_ADDBLOCK_POSITION_DEFAULT, BLOCK_ADDBLOCK_POSITION_FLATNAV.
+     * @var int
+     */
+    public $addblockposition;
+
+    /**
+     * editor_scss file(s) provided by this theme.
+     * @var array
+     */
+    public $editor_scss;
+
+    /**
+     * Name of the class extending \core\output\icon_system.
+     * @var string
+     */
+    public $iconsystem;
+
+    /**
+     * Theme defines its own editing mode switch.
+     * @var bool
+     */
+    public $haseditswitch = false;
+
     /**
      * Load the config.php file for a particular theme, and return an instance
      * of this class. (That is, this is a factory method.)