input: Write a function to count the number of words and characters in a string.
  assert word_and_character_count("Hello world") == (2, 11)
  assert word_and_character_count("Python programming is fun") == (4, 26)
  assert word_and_character_count("AI") == (1, 2)
output: def word_and_character_count(s):
      words = s.split()
      num_words = len(words)
      num_chars = len(s)
      return num_words, num_chars