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/errors.h b/src/errors.h
index 1c1b146..67012e7 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3490,5 +3490,9 @@
 	INIT(= N_("E1404: Positional argument %d type used inconsistently: %s/%s"));
 EXTERN char e_invalid_format_specifier_str[]
 	INIT(= N_("E1405: Invalid format specifier: %s"));
+EXTERN char e_member_str_type_mismatch_expected_str_but_got_str[]
+	INIT(= N_("E1406: Member \"%s\": type mismatch, expected %s but got %s"));
+EXTERN char e_method_str_type_mismatch_expected_str_but_got_str[]
+	INIT(= N_("E1407: Member \"%s\": type mismatch, expected %s but got %s"));
 
 // E1365 - E1399 unused