patch 9.0.1786: Vim9: need instanceof() function

patch 9.0.1786: Vim9: need instanceof() function

Problem:  Vim9: need instanceof() function
Solution: Implement instanceof() builtin

Implemented in the same form as Python's isinstance because it allows
for checking multiple class types at the same time.

closes: #12867

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: LemonBoy <thatlemon@gmail.com>