Implementing Character and Integer Stack using single Array

# Aim

- I want to contribute code to Implement Character and Integer stack using single array.

# Details

- I will contribute this in C it will be an implementation of 2 stacks using single array.

### Programming language

- [x] C
- [ ] C++
- [ ] Java
- [ ] Python

The issue names no target file, test, or entry point. Start by locating the repository’s C implementations and how stack examples are organized; done means a C implementation of character and integer stacks sharing one array, with a way to verify the behavior.

Written by the indexing model from the issue text.