input: Write a python function to count the number of vowels in a list of words.
  assert count_vowels(["hello", "world"]) == 2
  assert count_vowels(["apple", "banana", "cherry"]) == 6
  assert count_vowels(["sky", "why", "try"]) == 0
output: def count_vowels(words):
      vowels = "aeiouAEIOU"
      return sum(1 for word in words for char in word if char in vowels)