DefaultLocale should continue returning a canonicalized language tag

[anba]

From https://w3c.github.io/webdriver-bidi/#command-emulation-setLocaleOverride:

If IsStructurallyValidLanguageTag(emulated locale) returns false, return error with error code invalid argument.

ECMA-402 DefaultLocale has more requirements:

1. The language tag is also canonicalized. For example "EN" is structurally valid, but not in canonical case. Or "de-DD" is also structurally valid and even in canonical case, but still not a canonical language tag, because "DD" should be replaced with "DE".
2. The default locale must be also be in [[AvailableLocales]] of all Intl service constructors, see https://tc39.es/ecma402/#sec-internal-slots. For example if the locale override is "nds", but the implementation doesn't ship locale data for Low German, than DefaultLocale must not return "nds".
3. Furthermore language tags in [[AvailableLocales]] don't contain Unicode extension sequences, see https://tc39.es/ecma402/#available-locales-list. That means for example that "en-u-nu-latn" must be rejected as a locale override.