Add StackUsingLinkedList.java for #6715

[Kamal1023]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java
  Closes [FEATURE REQUEST] Add Stack Implementation Using Linked List in Java #6715
  This pull request adds a stack data structure implemented using a singly linked list. The StackUsingLinkedList class supports standard stack operations—push, pop, peek, isEmpty, and size - all in constant O(1) time complexity.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.69%. Comparing base (f0fb971) to head (a096583).

Files with missing lines	Patch %	Lines
...com/thealgorithms/stacks/StackUsingLinkedList.java	0.00%	28 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6740      +/-   ##
============================================
- Coverage     76.80%   76.69%   -0.12%     
  Complexity     6047     6047              
============================================
  Files           719      720       +1     
  Lines         20342    20370      +28     
  Branches       3940     3944       +4     
============================================
- Hits          15624    15622       -2     
- Misses         4127     4156      +29     
- Partials        591      592       +1     

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