Make a vl-warning-p and add it to a vl-delta-p.
This is just a convenient shorthand.
(dwarn &key (delta 'delta) type msg args fatalp (fn '__function__)) extends delta with a new warning with the given type, msg, etc.
Since delta defaults to 'delta, you can omit it in the common case that your vl-delta-p is called delta.
