function getAvailableLocales() {
  const localesWithMessages = new Set([...Object.keys(LOCALE_MESSAGES), DEFAULT_LOCALE]);
  return /** @type {Array<LH.Locale>} */ ([...localesWithMessages].sort());
}
Returns a list of available locales.
 - if full build, this includes all canonical locales, aliases, and any locale added
     via `registerLocaleData`.
 - if bundled and locale messages have been stripped (locales.js shimmed), this includes
     only DEFAULT_LOCALE and any locales from `registerLocaleData`.
@return {Array<LH.Locale>}