Checking if Alphabetic Sum is Even

# Introduction

In this lab, you will write a program that determines if the sum of the positions of the letters in the alphabet of a given string is even or odd, and defines a function named `is_sum_even()` that takes a single argument `string`, calculates the sum of the positions of all the letters in the alphabet of `string` (case-insensitive), ignores non-letter symbols in the string, and returns `True` if the sum is even and `False` if it is odd.