input: Write a function to split a string into substrings of a given length and join them with commas.
  assert split_string("abcdefghij", 3) == "abc,def,ghi,j"
  assert split_string("hello world", 2) == "he,ll,o ,wo,rl,d"
  assert split_string("python", 4) == "pyth,on"
output: def split_string(s, length):
      return ','.join(s[i:i+length] for i in range(0, s.length, length))