input: Write a python function to score the similarity between two strings.
  assert score_similarity("abc", "abc") == 0.5
  assert score_similarity("abc", "a") == 0.25
  assert score_similarity("abc", "def") == 0.0
  assert score_similarity("abc", "abcd") == 0.4
output: def score_similarity(str1, str2):
      common_chars = set(str1).intersection(set(str2))
      common_count = len(common_chars)
      total_unique_chars = len(set(str1 + str2))
      return common_count / total_unique_chars