patch 8.2.2511: Vim9: cannot use Vim9 script syntax in some places

Problem:    Vim9: cannot use Vim9 script syntax in some places.
Solution:   Add the :vim9cmd command modifier. Incompatible: Makes ":vim9"
            mean ":vim9cmd" instead of ":vim9script".
diff --git a/src/errors.h b/src/errors.h
index 927e1ff..05f90b1 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -363,3 +363,5 @@
 	INIT(= N_("E1162: Register name must be one character: %s"));
 EXTERN char e_variable_nr_type_mismatch_expected_str_but_got_str[]
 	INIT(= N_("E1163: Variable %d: type mismatch, expected %s but got %s"));
+EXTERN char e_vim9cmd_must_be_followed_by_command[]
+	INIT(= N_("E1164: vim9cmd must be followed by a command"));