input: Write a python function to split a string into a list of strings separated by commas.
  assert split_string("1, 2, 3, 4") == ['1', '2', '3', '4']
  assert split_string("apple, banana, cherry") == ['apple', 'banana', 'cherry']
  assert split_string("dog, cat, mouse") == ['dog', 'cat', 'mouse']
output: def split_string(s):
      return [item.strip() for item in s.split(',')]