input: Write a python function to return the unique characters from a string in the order they appear.
  assert unique_chars("hello") == ['h', 'e', 'l', 'o']
  assert unique_chars("banana") == ['b', 'a', 'n']
  assert unique_chars("abcabc") == ['a', 'b', 'c']
  assert unique_chars("aaabbbccc") == ['a', 'b', 'c']
output: def unique_chars(s):
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return result