patch 9.0.1883: Vim9: Calling an interface method using a child object fails

patch 9.0.1883: Vim9: Calling an interface method using a child object fails

Problem:  Vim9: Calling an interface method using a child object fails
Solution: Search methods of parent class

When a class implementing an interface is extended by another class and
a child class instance is passed to a function that accepts the
interface, calling an interface method doesn't work properly.

closes: #13053

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