Refactor stack implementation using global top pointer

[IshuSinghSE]

This pull request refactors the stack implementation in Program.c from a function-based approach using root pointers to a simpler global-pointer design and introduces a menu-driven interface for user interaction. The changes improve usability and code clarity by consolidating stack operations and providing interactive options.

Refactoring and simplification of stack operations:

• Replaced the previous function-based stack implementation (using root pointers and helper functions like newNode, isEmpty, and peek) with a global top pointer and direct stack manipulation functions: push, pop, and display.
• Simplified push and pop functions to operate directly on the global top pointer, removing the need to pass pointers and return values.

User interface improvements:

• Added a menu-driven loop in main to allow users to interactively push, pop, and display stack elements, with clear prompts and feedback for each operation.
• Implemented a new display function to show all stack elements, enhancing visibility of stack contents during program execution.

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