feat: Added topological sort using DFS

[mjk22071998]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.16%. Comparing base (14a23b7) to head (397e9a9).

Files with missing lines	Patch %	Lines
...ithms/datastructures/trees/BinaryTreeToString.java	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6757      +/-   ##
============================================
+ Coverage     77.11%   77.16%   +0.04%     
- Complexity     6111     6130      +19     
============================================
  Files           719      721       +2     
  Lines         20437    20483      +46     
  Branches       3952     3962      +10     
============================================
+ Hits          15761    15805      +44     
- Misses         4079     4080       +1     
- Partials        597      598       +1     

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[DenizAltunkapan]

It looks like this implementation duplicates the functionality of the existing TopologicalSort class under com.thealgorithms.sorts.

That implementation already provides a more complete and robust DFS-based topological sorting algorithm.

Given that, this new version in com.thealgorithms.graph seems redundant.
Unless it adds significant improvements or new behavior, it would be better to reuse or refactor the existing TopologicalSort class instead of introducing a parallel implementation.

Next time please check our Directory.md if your proposed class already exists.
@mjk22071998