Add Topological Sorting using DFS (Issue #6938)

[Senrian]

Description

Implements topological sorting using Depth-First Search (DFS) for TheAlgorithms/Java repository, addressing issue #6938.

Algorithm

Given a Directed Acyclic Graph (DAG), the algorithm:

1. Performs DFS traversal on all unvisited vertices
2. Records each vertex in post-order (when all descendants have been explored)
3. Reverses the recorded order to produce the topological ordering

Complexity

• Time Complexity: O(V + E) where V = vertices, E = edges
• Space Complexity: O(V + E) for adjacency list and recursion stack

Files

• TopologicalSort.java - DFS-based topological sort implementation
• TopologicalSortTest.java - Comprehensive tests

Closes #6938