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/structs.h

@@ -4764,14 +4764,22 @@ typedef enum {
     MAGIC_ALL = 4		// "\v" very magic
 } magic_T;
 
+typedef enum {
+    WT_UNKNOWN = 0,	// Unknown or unspecified location
+    WT_ARGUMENT,
+    WT_VARIABLE,
+    WT_MEMBER,
+    WT_METHOD,
+} wherekind_T;
+
 // Struct used to pass to error messages about where the error happened.
 typedef struct {
-    char    *wt_func_name;  // function name or NULL
-    char    wt_index;	    // argument or variable index, 0 means unknown
-    char    wt_variable;    // "variable" when TRUE, "argument" otherwise
+    char	*wt_func_name;  // function name or NULL
+    char	wt_index;	// argument or variable index, 0 means unknown
+    wherekind_T	wt_kind;	// "variable" when TRUE, "argument" otherwise
 } where_T;
 
-#define WHERE_INIT {NULL, 0, 0}
+#define WHERE_INIT {NULL, 0, WT_UNKNOWN}
 
 // Struct passed to get_v_event() and restore_v_event().
 typedef struct {