input: Write a function to find the longest word in a list and return its length.
  assert longest_word(["laptop", "computer", "keyboard"]) == ("keyboard", 8)
  assert longest_word(["apple", "banana", "pear"]) == ("banana", 6)
  assert longest_word(["a", "ab", "abc"]) == ("abc", 3)
output: def longest_word(words):
      longest = max(words, key=len)
      return longest, len(longest)