patch 9.1.1094: Vim9: problem finding implemented method in type hierarchy

patch 9.1.1094: Vim9: problem finding implemented method in type hierarchy

Problem:  Vim9: problem finding implemented method for abstract method
          in type hierarchy (Aliaksei Budavei)
Solution: When checking for abstract methods in an extended class, check
          whether an abstract method is implemented in one of the parent
          classes (Yegappan Lakshmanan)

fixes: #16495
closes: #16497

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