feat: add StackUsingLinkedList implementation with tests

[prashantpiyush1111]

Added StackUsingLinkedList implementation using a singly linked list.

Features:

• Supports push, pop, peek, and isEmpty operations
• Proper documentation with explanations
• Unit tests covering normal and edge cases

This improves the stack data structure collection and ensures correctness through test cases.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.51%. Comparing base (79bc620) to head (f330d39).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7382      +/-   ##
============================================
+ Coverage     79.50%   79.51%   +0.01%     
- Complexity     7161     7170       +9     
============================================
  Files           797      798       +1     
  Lines         23409    23426      +17     
  Branches       4603     4603              
============================================
+ Hits          18611    18628      +17     
  Misses         4054     4054              
  Partials        744      744              

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