events: define event handler as enumerable

PR-URL: https://github.com/nodejs/node/pull/35931


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>