Update Keywords to include class_name

Includes `class_name` in the keywords list, and updates the definition of `class`.

getting_started/scripting/gdscript/gdscript_basics.rst

@@ -160,7 +160,9 @@ in case you want to take a look under the hood.
 +------------+---------------------------------------------------------------------------------------------------------------+
 | return     | Returns a value from a function.                                                                              |
 +------------+---------------------------------------------------------------------------------------------------------------+
-| class      | Defines a class.                                                                                              |
+| class      | Defines an inner class.                                                                                       |
++------------+---------------------------------------------------------------------------------------------------------------+
+| class_name | Defines a class name and optional icon for your script.                                                       |
 +------------+---------------------------------------------------------------------------------------------------------------+
 | extends    | Defines what class to extend with the current class.                                                          |
 +------------+---------------------------------------------------------------------------------------------------------------+