input: Write a python function to combine two words by mixing their letters alternatively.
  assert mix_words("abc", "xyz") == "axbycz"
  assert mix_words("hello", "world") == "hweolrllod"
  assert mix_words("short", "longer") == "slhoonrgter"
  assert mix_words("mix", "match") == "mmiaxtch"
output: def mix_words(word1, word2):
      combined = ''.join([a + b for a, b in zip(word1, word2)])
      combined += word1[len(word2):] + word2[len(word1):]
      return combined