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.
GetMiddle("test") ➞ "es"
GetMiddle("testing") ➞ "t"
GetMiddle("middle") ➞ "dd"
GetMiddle("A") ➞ "A"All test cases contain a single word (as a string).