Given an integer n, return a butterfly pattern of size n. The top half has n rows: row i is i stars, then (2n - 2i) spaces, then i stars. The bottom half mirrors the top, giving a total of 2n rows joined by \n.
Example: n = 3 →
* *
** **
******
******
** **
* *
3
* * ** ** ****** ****** ** ** * *