input: Write a function that takes two strings and returns true if the first string is a subsequence of the second.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("abc", "qaxcy")==False
  assert is_subsequence("abc", "bcab")==True
  assert is_subsequence("", "qaxbcy")==True
  assert is_subsequence("abc", "")==False
output: def is_subsequence(s1, s2):
      i, j = 0, 0
      while i < len(s1) and j < len(s2):
          if s1[i] == s2[j]:
              i += 1
          j += 1
      return i == len(s1)