String Calculator

Goal: Create a calculator that takes a variable number of strings, converts them to integers, and returns the sum

1. Create a StringCalculator class with an add() method. The add method should return 0 when the input is empty

StringCalculator.add() --> 0

2. add(number) should return the number as an integer

StringCalculator.add('12') --> 12

3. add(a, b) should return the sum of the numbers

StringCalculator.add('7', '4') --> 11

4. add(n1, n2, n3, ...ni) should return the sum of the numbers

StringCalculator.add('1', '3', 5', '2') --> 11

4. Strings should be parsed by IntegerParser.parse(String number)