refactor: PasswordGen

[alxkm]

Refactor PasswordGen.
Adding 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 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 46.80%. Comparing base (4e72056) to head (18f6018).

Files	Patch %	Lines
...ain/java/com/thealgorithms/others/PasswordGen.java	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5373      +/-   ##
============================================
- Coverage     46.82%   46.80%   -0.02%     
- Complexity     2898     2900       +2     
============================================
  Files           525      525              
  Lines         15376    15373       -3     
  Branches       2910     2910              
============================================
- Hits           7200     7196       -4     
  Misses         7877     7877              
- Partials        299      300       +1     

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