patch 8.2.3179: Vim9: cannot assign to an imported variable at script level

Problem:    Vim9: cannot assign to an imported variable at script level.
Solution:   Lookup imported items when assigning.
diff --git a/src/errors.h b/src/errors.h
index 35d1487..be1341a 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -506,3 +506,5 @@
 	INIT(= N_("E1211: List required for argument %d"));
 EXTERN char e_bool_required_for_argument_nr[]
 	INIT(= N_("E1211: Bool required for argument %d"));
+EXTERN char e_redefining_imported_item_str[]
+	INIT(= N_("E1212: Redefining imported item %s"));