Intl.DateTimeFormat does not support 'und' locale

[sven-oly]

This seems wrong. Apparently 'und' falls back to 'en', which is different behavior than ICU4C.

Examples:

Welcome to Node.js v18.19.1.
Type ".help" for more information.
> dt = new Intl.DateTimeFormat('und', {"month":"short","weekday":"narrow","day":"numeric","calendar":"gregory","numberingSystem":"latn"})
DateTimeFormat [Intl.DateTimeFormat] {}
> dt.format()
'T, Apr 30'

> Intl.DateTimeFormat.supportedLocalesOf(["und"])
[]
> Intl.DateTimeFormat.supportedLocalesOf(["und", "en"])
[ 'en' ]