refactor: QueueUsingTwoStacks

[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

All modified and coverable lines are covered by tests ✅

Project coverage is 50.15%. Comparing base (203d544) to head (def0979).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5427      +/-   ##
============================================
+ Coverage     49.97%   50.15%   +0.18%     
- Complexity     3108     3120      +12     
============================================
  Files           524      524              
  Lines         15166    15143      -23     
  Branches       2884     2878       -6     
============================================
+ Hits           7579     7595      +16     
+ Misses         7271     7230      -41     
- Partials        316      318       +2     

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