input: Write a function to detect the first vowel in a given string. If the string contains a vowel, return the first vowel found. If the string does not contain any vowels, return None.
  assert vowelCheck("Hello World")=="e"
  assert vowelCheck("Luk")==None
  assert vowelCheck("Apple")=="A"
  assert vowelCheck("bcdfg")==None
  assert vowelCheck("IoU")=="I"
output: def vowelCheck(s):
      vowels = "AEIOUaeiou"
      for char in s:
          if char in vowels:
              return char
      return None