input: Write a python function to split a string by comma and space.
  assert split_string("apple, banana, orange, grapes") == ["apple", "banana", "orange", "grapes"]
  assert split_string("cat, dog, bird, fish") == ["cat", "dog", "bird", "fish"]
  assert split_string("red, blue, green, yellow") == ["red", "blue", "green", "yellow"]
output: def split_string(s):
      return s.split(', ')