input: Write a function to filter strings that start with a vowel.
  assert filter_vowel_strings(["apple", "banana", "orange", "umbrella", "grape"]) == ["apple", "orange", "umbrella"]
  assert filter_vowel_strings(["elephant", "tiger", "iguana", "ostrich", "cat"]) == ["elephant", "iguana", "ostrich"]
  assert filter_vowel_strings(["dog", "cat", "fish", "antelope", "eagle"]) == ["antelope", "eagle"]
output: def filter_vowel_strings(strings):
      vowels = 'aeiouAEIOU'
      return [s for s in strings if s[0] in vowels]