feat: Add PostfixEvaluator new algorithm with Junit tests

[Hardvan]

• 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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.

Project coverage is 64.09%. Comparing base (596c614) to head (02c8728).

Files with missing lines	Patch %	Lines
...ava/com/thealgorithms/stacks/PostfixEvaluator.java	84.21%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5754      +/-   ##
============================================
+ Coverage     64.05%   64.09%   +0.04%     
- Complexity     4177     4187      +10     
============================================
  Files           584      585       +1     
  Lines         16332    16351      +19     
  Branches       3152     3155       +3     
============================================
+ Hits          10461    10480      +19     
- Misses         5445     5446       +1     
+ Partials        426      425       -1     

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