Each year has 365 or 366 days. Given a string date representing a Gregorian calendar date formatted as month/day/year, return the day-number of the year.
day_of_year("11/16/2020") ➞ 321
day_of_year("1/9/2019") ➞ 9
day_of_year("3/1/2004") ➞ 61
day_of_year("12/31/2000") ➞ 366
All input strings in tests are valid dates.