/**
 * Resolve the ISO day-of-week number (1 = Monday .. 7 = Sunday) that a
 * locale considers the first day of the week, via
 * `Intl.Locale.prototype.weekInfo`.
 *
 * - Returns `null` if `locale` is not a valid BCP 47 tag.
 * - Falls back to `1` (Monday, matching GMT's existing ISO default in
 *   `startOfDate`/`startOfZoned`) if `weekInfo` is unavailable on the
 *   runtime (older engines) or unresolvable for the given locale.
 */
export declare function getLocaleFirstDayOfWeek(locale: string): number | null;