patch 8.2.2744: Vim9: no way to explicitly ignore an argument

Problem:    Vim9: no way to explicitly ignore an argument.
Solution:   Use the underscore as the name for an ignored argument.
diff --git a/src/evalvars.c b/src/evalvars.c
index 67abdcb..5869a82 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -3188,6 +3188,11 @@
 	goto failed;
     }
     var_in_vim9script = is_script_local && current_script_is_vim9();
+    if (var_in_vim9script && name[0] == '_' && name[1] == NUL)
+    {
+	emsg(_(e_cannot_use_underscore_here));
+	goto failed;
+    }
 
     di = find_var_in_ht(ht, 0, varname, TRUE);