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