input: Write a python function to find the longest common subsequence between two strings.
  assert longest_common_subsequence("abcde", "ace") == ("ace", 3)
  assert longest_common_subsequence("abc", "abc") == ("abc", 3)
  assert longest_common_subsequence("abc", "def") == ("", 0)
  assert longest_common_subsequence("AGGTAB", "GXTXAYB") == ("GTAB", 4)
output: def longest_common_subsequence(s1, s2):
      m, n = len(s1), len(s2)
      dp = [["" for _ in range(n + 1)] for _ in range(m + 1)]
  
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if s1[i - 1] == s2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + s1[i - 1]
              else:
                  if len(dp[i - 1][j]) > len(dp[i][j - 1]):
                      dp[i][j] = dp[i - 1][j]
                  else:
                      dp[i][j] = dp[i][j - 1]
  
      lcs = dp[m][n]
      return lcs, len(lcs)