patch 8.2.2331: Vim9: wrong error when modifying dict declared with :final

Problem:    Vim9: wrong error when modifying dict declared with :final.
Solution:   Do not check for writable variable when an index follows.
            (closes #7657)

src/structs.h

@@ -1777,7 +1777,7 @@ struct svar_S {
     char_u	*sv_name;	// points into "sn_all_vars" di_key
     typval_T	*sv_tv;		// points into "sn_vars" or "sn_all_vars" di_tv
     type_T	*sv_type;
-    int		sv_const;
+    int		sv_const;	// 0, ASSIGN_CONST or ASSIGN_FINAL
     int		sv_export;	// "export let var = val"
 };