String Converter: Implement a program that asks the user to input up to 20 strings (stopping when the user inputs 0). Store
these strings in a 2-dimensional array. Replace all the characters that are upper-cased to their lower-case equivalent.
Similarly, replace all the characters that are lower-cased to their upper-case equivalent.
Requirement: Implement a function that converts a character's case. You cannot use existing functions in the C-runtime
library for upper-casing/lower-casing. This function should take a char as the input and return a char (the same character
with its case converted).
char convertChar (char a)