input: Write a function to add a string to a list.
  assert add_string_to_list(['apple', 'orange', 'banana'], 'kiwi') == ['apple', 'orange', 'banana', 'kiwi']
  assert add_string_to_list([], 'strawberry') == ['strawberry']
  assert add_string_to_list(['cherry'], 'mango') == ['cherry', 'mango']
output: def add_string_to_list(lst, string):
      lst.append(string)
      return lst