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/vim9cmds.c b/src/vim9cmds.c
index c3c900a..29ed054 100644
--- a/src/vim9cmds.c
+++ b/src/vim9cmds.c
@@ -1045,8 +1045,11 @@
 		}
 
 		// Reserve a variable to store "var".
-		where.wt_index = var_list ? idx + 1 : 0;
-		where.wt_variable = TRUE;
+		if (var_list)
+		{
+		    where.wt_index = idx + 1;
+		    where.wt_kind = WT_VARIABLE;
+		}
 		if (lhs_type == &t_any)
 		    lhs_type = item_type;
 		else if (item_type != &t_unknown