Feat/add postman sort

[PauLopNun]

• 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.
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[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.61%. Comparing base (69d8406) to head (6f16064).

Files with missing lines	Patch %	Lines
...main/java/com/thealgorithms/sorts/PostmanSort.java	81.81%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #6665   +/-   ##
=========================================
  Coverage     76.60%   76.61%           
- Complexity     6026     6034    +8     
=========================================
  Files           719      720    +1     
  Lines         20326    20348   +22     
  Branches       3937     3942    +5     
=========================================
+ Hits          15571    15589   +18     
- Misses         4164     4165    +1     
- Partials        591      594    +3     

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[DenizAltunkapan]

@PauLopNun
The current implementation is named PostmanSort, but the code actually implements a Shellsort-like algorithm with a gap factor of 1.3. While it correctly sorts the array, it does not implement the classical Postman Sort as described in the reference link, which is typically a digit- or bucket-based sorting algorithm (similar to counting sort or radix sort for postal codes).

As a result, the name PostmanSort is misleading. To address this, you could either:

1. Rename the class to ShellSort (or another appropriate name), since a Shellsort implementation already exists here.
2. Implement a true Postman Sort, following a digit/bucket-based approach.

Nevertheless, this PR cannot be merged, because the implementation does not match the algorithm name or description.