Add Infix to Postfix conversion using Stack

[laiba-Ashfaq]

Fixes #6638

This program converts infix expressions to postfix notation using a stack. It includes operator precedence handling and supports basic arithmetic operators.
Input:
String infix = "A+BC-D";
Output:
Infix Expression: A+BC-D
Postfix Expression: ABC*+D-

• 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.
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• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.74%. Comparing base (ade777a) to head (caf2976).

Additional details and impacted files

@@                 Coverage Diff                 @@
##             update-directory    #6643   +/-   ##
===================================================
  Coverage               75.74%   75.74%           
  Complexity               5766     5766           
===================================================
  Files                     701      701           
  Lines                   19757    19757           
  Branches                 3831     3831           
===================================================
  Hits                    14965    14965           
  Misses                   4214     4214           
  Partials                  578      578           

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