input: Write a function to find the longest consecutive sequence of identical letters in a string.
  assert longest_consecutive_letters("aaabbbaacccdddde") == "dddd"
  assert longest_consecutive_letters("abcde") == "a"
  assert longest_consecutive_letters("aabbccddeeffggggh") == "gggg"
output: def longest_consecutive_letters(s):
      if not s:
          return ""
      
      max_letter = s[0]
      max_length = 1
      current_letter = s[0]
      current_length = 1
      
      for i in range(1, s.length):
          if s[i] == s[i - 1]:
              current_length += 1
          else:
              if current_length > max_length:
                  max_length = current_length
                  max_letter = current_letter
              current_letter = s[i]
              current_length = 1
      
      if current_length > max_length:
          max_letter = current_letter
      
      return max_letter * max_length