input: Write a function that accepts a string as an input and returns the total number of vowels in the string. If the string is empty then return 0.
  assert count_vowels("hello") == 2
  assert count_vowels("world") == 1
  assert count_vowels("aeiou") == 5
  assert count_vowels("bcdfghjkl") == 0
  assert count_vowels("") == 0
output: def count_vowels(s):
      vowels = "aeiou"
      return sum(1 for char in s if char in vowels)