static getStartOfWeek({ locale = null, locObj = null } = {}) {
    return (locObj || Locale.create(locale)).getStartOfWeek();
  }
Get the weekday on which the week starts according to the given locale.
@param {Object} opts - options
@param {string} [opts.locale] - the locale code
@param {string} [opts.locObj=null] - an existing locale object to use
@returns {number} the start of the week, 1 for Monday through 7 for Sunday