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>

src/eval.c

@@ -3859,7 +3859,6 @@ eval8(
 		{
 		    where_T where = WHERE_INIT;
 
-		    where.wt_variable = TRUE;
 		    res = check_type(want_type, actual, TRUE, where);
 		}
 	    }