input: Write a function to find the longest word in a list of words.
  assert longest_word(["Hello", "how", "are", "you"]) == "Hello"
  assert longest_word(["apple", "banana", "cherry"]) == "banana"
  assert longest_word(["short", "tiny", "small", "miniscule"]) == "miniscule"
output: def longest_word(words):
      return max(words, key=len)