Given a letter, created a function which returns the nearest vowel to the letter. If two vowels are equidistant to the given letter, return the earlier vowel.
nearestVowel("b") ➞ "a"
nearestVowel("s") ➞ "u"
nearestVowel("c") ➞ "a"
nearestVowel("i") ➞ "i"