input: Write a python function to convert a string into a list of its characters.
  assert string_to_char_list("hello")==['h', 'e', 'l', 'l', 'o']
  assert string_to_char_list("world")==['w', 'o', 'r', 'l', 'd']
  assert string_to_char_list("python")==['p', 'y', 't', 'h', 'o', 'n']
output: def string_to_char_list(s):
      return list(s)