Write a function that takes all even-indexed characters and odd-indexed characters from a string and concatenates them together.
To illustrate:
indexShuffle("abcd") ➞ "acbd"
// "ac" (even-indexed) + "bd" (odd-indexed)
indexShuffle("abcdefg") ➞ "acegbdf"
indexShuffle("holiday") ➞ "hldyoia"
indexShuffle("maybe") ➞ "myeab"
0 should be treated as an even number.