patch 9.0.1737: Calling a base class method through an extended class fails

patch 9.0.1737: Calling a base class method through an extended class fails

Problem: Calling a base class method through an extended class fails
Solution: Create lookup table for member index in the interface to
          to the member class implementing the interface

Create additional tests for Vim9 classes.  Fix unconvered memory leaks
and crashes found by the new tests.

closes: #12848
closes: #12089

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>author