Create a function that takes a word and extends all vowels by a number num.
extendVowels("Hello", 5) ➞ "Heeeeeelloooooo"
extendVowels("Innokodakademija", 3) ➞ "EEEEdaaaabiiiit"
extendVowels("Extend", 0) ➞ "Extend"
Return "invalid" if num is not a positive integer or 0.