input: Write a function to count the total number of vowels in a given string. If the given string is empty then return 0.
  assert count_vowels("hello world") == 3
  assert count_vowels("python programming") == 4
  assert count_vowels("aeiou") == 5
  assert count_vowels("bcdfghjklmnpqrstvwxyz") == 0
  assert count_vowels("") == 0
output: def count_vowels(s):
      vowels = "aeiouAEIOU"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count