verify no locale leaks in ESR60 `Intl` APIs

​Several new Intl APIs and enhancements to existing APIs were added during the ESR60 development cycle. We should review the changes to make sure locale info, etc. is not leaked when privacy.resistFingerprinting is true. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1403318 ​https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/PluralRules

​https://bugzilla.mozilla.org/show_bug.cgi?id=1403319 ​https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NumberFormat/formatToParts

​https://bugzilla.mozilla.org/show_bug.cgi?id=1386146 ​https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat