get weeksInWeekYear() {
    return this.isValid ? weeksInWeekYear(this.weekYear) : NaN;
  }
Returns the number of weeks in this DateTime's year
@see https://en.wikipedia.org/wiki/ISO_week_date
@example DateTime.local(2004).weeksInWeekYear //=> 53
@example DateTime.local(2013).weeksInWeekYear //=> 52
@type {number}