input: Write a function to check if the first string is a subsequence of the second string. If it is a subsequence return True else return False.
  assert is_subsequence("abc","qaxbcy")==True
  assert is_subsequence("abc","aaxbby")==True
  assert is_subsequence("xyz","xyyzzzxy")==True
  assert is_subsequence("abs","abxxsxy")==False
  assert is_subsequence("","abc")==True
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)