input: Write a python function to find the longest word in a list of words.
  assert longest_word(["apple", "banana", "cherry", "date"]) == "banana"
  assert longest_word(["word", "another", "supercalifragilisticexpialidocious", "tiny"]) == "supercalifragilisticexpialidocious"
  assert longest_word(["one", "two", "three", "four"]) == "three"
output: def longest_word(words):
      return max(words, key=len)