Given a square list (n*n size) implement a function that returns a new list containing two lists equal to the two diagonals, in the following order:
diagonal 1 = from upper-left to lower-right corner
diagonal 2 = from upper-right to lower-left corner
get_diagonals([ [1, 2], [3, 4] ]) ➞ [ [1, 4], [2, 3] ]
get_diagonals([ ["a", "b", "c"], ["d", "e", "f"], ["g", "h", "i"] ]) ➞ [ ["a", "e", "i"], ["c", "e", "g"] ]
get_diagonals([ [True] ]) ➞ [ [True], [True] ]