input: Write a function to append a new string to a list of strings.
  assert append_to_list(["Hello", "World"], "Hi") == ["Hello", "World", "Hi"]
  assert append_to_list(["Python", "Java"], "C++") == ["Python", "Java", "C++"]
  assert append_to_list([], "First") == ["First"]
output: def append_to_list(lst, string):
      lst.append(string)
      return lst