refactor: cleanup ShellSort

[alxkm]

Improving ShellSort code readability
Refactor the code by moving parts of it into well-named methods to improve readability and maintainability.
Adopt a more general testing approach to ensure better test coverage.

• 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 41.47%. Comparing base (ebed8b3) to head (0d17676).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5302      +/-   ##
============================================
+ Coverage     41.41%   41.47%   +0.05%     
- Complexity     2551     2554       +3     
============================================
  Files           520      520              
  Lines         15447    15447              
  Branches       2950     2947       -3     
============================================
+ Hits           6398     6406       +8     
+ Misses         8754     8746       -8     
  Partials        295      295              

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[siriak]

Looks good, thanks!