patch 8.2.1894: Vim9: command modifiers are not supported

Problem:    Vim9: command modifiers are not supported.
Solution:   Support "silent" and "silent!".
diff --git a/src/evalvars.c b/src/evalvars.c
index b3ad2c9..1f418ae 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -3577,9 +3577,11 @@
     tv.v_type = VAR_STRING;
     tv.vval.v_string = (char_u *)"";
     if (append)
-	set_var_lval(redir_lval, redir_endp, &tv, TRUE, 0, (char_u *)".");
+	set_var_lval(redir_lval, redir_endp, &tv, TRUE,
+						ASSIGN_NO_DECL, (char_u *)".");
     else
-	set_var_lval(redir_lval, redir_endp, &tv, TRUE, 0, (char_u *)"=");
+	set_var_lval(redir_lval, redir_endp, &tv, TRUE,
+						ASSIGN_NO_DECL, (char_u *)"=");
     clear_lval(redir_lval);
     if (called_emsg > called_emsg_before)
     {