patch 8.2.1744: Vim9: using ":const!" is weird

Problem:    Vim9: using ":const!" is weird.
Solution:   Use "var" - "final" - "const" like Dart.  "let" still works for
            now.
diff --git a/src/errors.h b/src/errors.h
index 0246fa1..34c47ca 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -270,4 +270,8 @@
 	INIT(= N_("E1122: Variable is locked: %s"));
 EXTERN char e_missing_comma_before_argument_str[]
 	INIT(= N_("E1123: Missing comma before argument: %s"));
+EXTERN char e_str_cannot_be_used_in_legacy_vim_script[]
+	INIT(= N_("E1124: \"%s\" cannot be used in legacy Vim script"));
+EXTERN char e_final_requires_a_value[]
+	INIT(= N_("E1125: Final requires a value"));
 #endif