refactor: LongestPalindromicSubstring

[alxkm]

Class cleanup.
Refactored javadocs.
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 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 49.77%. Comparing base (45563cc) to head (1e92a2f).

Files with missing lines	Patch %	Lines
...ynamicprogramming/LongestPalindromicSubstring.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5420      +/-   ##
============================================
+ Coverage     49.64%   49.77%   +0.13%     
- Complexity     3078     3090      +12     
============================================
  Files           524      524              
  Lines         15195    15188       -7     
  Branches       2890     2890              
============================================
+ Hits           7543     7560      +17     
+ Misses         7337     7314      -23     
+ Partials        315      314       -1     

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