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>
diff --git a/src/evalfunc.c b/src/evalfunc.c
index c0710da..8561e94 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -647,6 +647,7 @@
 	    t_func_exp.tt_argcount = -1;
 
 	where.wt_index = 2;
+	where.wt_kind = WT_ARGUMENT;
 	return check_type(&t_func_exp, type, TRUE, where);
     }
     return OK;
@@ -714,6 +715,7 @@
 	    if (type->tt_argcount == -1)
 		t_func_exp.tt_argcount = -1;
 	    where.wt_index = 2;
+	    where.wt_kind = WT_ARGUMENT;
 	    return check_type(&t_func_exp, type, TRUE, where);
 	}