ensure no locale leaks from new Intl APIs

From #33534 (closed):

Firefox 76 enabled the following:

• numberingSystem and calendar options for the Intl.NumberFormat, Intl.DateTimeFormat, and Intl.RelativeTimeFormat constructors
• the Intl.ListFormat API

Firefox 78 added support for Intl.DisplayNames.

It looks like RelativeTimeFormat() may leak the current locale if the caller passes undefined to the constructor instead of a locale string. Other APIs may have similar problems that could make it easier to perform fingerprinting based on the user's locale.

https://bugzilla.mozilla.org/show_bug.cgi?id=1625975
"Enable numberingSystem and calendar options by default"

https://bugzilla.mozilla.org/show_bug.cgi?id=1589095
"Add support for "type" and "style" options to Intl.ListFormat and enable Intl.ListFormat by default"

https://bugzilla.mozilla.org/show_bug.cgi?id=1557727
"Implement the Intl.DisplayNames proposal"