code: function normalizeUnit(unit) {
    const normalized = {
      year: "year",
      years: "year",
      month: "month",
      months: "month",
      day: "day",
      days: "day",
      hour: "hour",
      hours: "hour",
      minute: "minute",
      minutes: "minute",
      second: "second",
      seconds: "second",
      millisecond: "millisecond",
      milliseconds: "millisecond",
      weekday: "weekday",
      weekdays: "weekday",
      weeknumber: "weekNumber",
      weeksnumber: "weekNumber",
      weeknumbers: "weekNumber",
      weekyear: "weekYear",
      weekyears: "weekYear",
      ordinal: "ordinal"
    }[unit.toLowerCase()];
  
    if (!normalized) throw new InvalidUnitError(unit);
  
    return normalized;
  }
docs: standardize case and plurality in units