input: Write a python function to extract the first and last letters of each word in a list.
  assert extract_letters(["cat", "dog", "snake", "elephant"]) == (['c', 'd', 's', 'e'], ['t', 'g', 'e', 't'])
  assert extract_letters(["apple", "banana", "cherry"]) == (['a', 'b', 'c'], ['e', 'a', 'y'])
  assert extract_letters(["word", "another", "last"]) == (['w', 'a', 'l'], ['d', 'r', 't'])
output: def extract_letters(words):
      first_letters = [word[0] for word in words]
      last_letters = [word[-1] for word in words]
      return first_letters, last_letters