Document extending scripted classes in scripts

[kiryvajiravuth]

Closes #51

Adds a new "Extending Scripted Classes" section to the Scripted Classes
article (Advanced/02.ScriptedClasses.md).

Since update 0.8.2, scripted classes can extend other scripted classes
rather than only native scriptable classes. The section documents:

• A worked example with a reusable scripted base class (MyBaseChar.hxc)
  and a subclass that extends it (MyChar.hxc), including override and
  super usage.
• Key rules:
  • A defined new() must call super(); a missing constructor
    auto-calls the superclass constructor.
  • Instance fields that collide with a superclass field throw an error.
  • Scripted functions/fields inherit through the whole chain of scripted
    superclasses.
  • Extending native scriptable classes still works as before.