input: Write a python function to encode a string using ROT13 cipher.
  assert rot13_encode("attack at dawn", 13) == "nggnpx ng qnja"
  assert rot13_encode("hello world", 13) == "uryyb jbeyq"
  assert rot13_encode("ROT13 Test", 13) == "EBG13 Grfg"
output: def rot13_encode(text, shift):
      def shift_char(c):
          if 'a' <= c <= 'z':
              return chr((ord(c) - ord('a') + shift) % 26 + ord('a'))
          elif 'A' <= c <= 'Z':
              return chr((ord(c) - ord('A') + shift) % 26 + ord('A'))
          else:
              return c
  
      return ''.join(shift_char(c) for c in text)