Given a number between 1-26, return what letter is at that position in the alphabet. Return 0 if the number given is not within that range, or isn't an integer.
letterAtPosition(1) ➞ "a"
letterAtPosition(26.0) ➞ "z"
letterAtPosition(0) ➞ "0"
letterAtPosition(4.5) ➞ "0"".0" are valid.