patch 9.0.1741: No type checking in interfaces

patch 9.0.1741: No type checking in interfaces

Problem: No type checking in interfaces
Solution: Implement member type check in vim9 interfaces

Most of the code is a small refactoring to allow the use of a where_T
for signaling the type mismatch, the type checking itself is pretty
simple.

Improve where_T error reports

Let the caller explicitly define the kind of location it's referring to
and free the WT_ARGUMENT enum from its catch-all role.

Implement type checking for interface methods

Follows closely the logic used for type-checking the members.

closes: #12844

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