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>

runtime/doc/usr_41.txt

@@ -877,6 +877,7 @@ Other computation:					*bitwise-function*
 	srand()			initialize seed used by rand()
 
 Variables:						*var-functions*
+	instanceof()		check if a variable is an instance of a given class
 	type()			type of a variable as a number
 	typename()		type of a variable as text
 	islocked()		check if a variable is locked