input: Write a function to split a given string by every nth character.
  assert splitStringByN("Hello World!", 3) == ['Hel', 'lo ', 'Wor', 'ld!']
  assert splitStringByN("abcdefg", 2) == ['ab', 'cd', 'ef', 'g']
  assert splitStringByN("123456789", 4) == ['1234', '5678', '9']
output: def splitStringByN(s, n):
      return [s[i:i+n] for i in range(0, len(s), n)]