refactor: PostfixToInfix

[alxkm]

Class cleanup.
Readability improvements.
Added tests.

• 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 70.58824% with 5 lines in your changes missing coverage. Please review.

Project coverage is 49.36%. Comparing base (fc5a70e) to head (32c6b62).

Files	Patch %	Lines
.../java/com/thealgorithms/stacks/PostfixToInfix.java	70.58%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5409      +/-   ##
============================================
+ Coverage     49.12%   49.36%   +0.23%     
- Complexity     3052     3067      +15     
============================================
  Files           524      524              
  Lines         15231    15204      -27     
  Branches       2898     2890       -8     
============================================
+ Hits           7482     7505      +23     
+ Misses         7442     7388      -54     
- Partials        307      311       +4     

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