Create a function that divides a string into parts of size n.
partition("chew", 2) ➞ ["ch", "ew"]
partition("thematic", 4) ➞ ["them", "atic"]
partition("c++", 2) ➞ ["c+", "+"]
For inputs that do not split evenly into N-sized parts, the last element in the array will have a "leftover" string (see example #3).