input: Write a python function to split a string at the first occurrence of a given substring.
  assert split_string('hello world', ' ') == ['hello', 'world']
  assert split_string('abc-def-ghi', '-') == ['abc', 'def-ghi']
  assert split_string('one,two,three', ',') == ['one', 'two,three']
output: def split_string(s, delimiter):
      split_index = s.find(delimiter)
      if split_index == -1:
          return [s]
      return [s[:split_index], s[split_index + len(delimiter):]]