Create a function that takes a string and returns the middle character(s). If the word's length is odd, return the middle character. If the word's length is even, return the middle two characters.
get_middle("test") ➞ "es"
get_middle("testing") ➞ "t"
get_middle("middle") ➞ "dd"
get_middle("A") ➞ "A"
All test cases contain a single word (as a string).