Added reverse() method to LinkedList

[RohiniShankari]

This PR introduces a new reverse() method to the LinkedList class.
Functionality:

• Reverses the order of nodes in the list in-place.
• Handles edge cases: empty lists and single-node lists.
• Iterates through the list, flipping the .next pointers of each node.
• Returns the LinkedList instance for method chaining.
  Testing:
  Added comprehensive test cases for:
• Normal multi-node lists
• Empty lists
• Single-node lists

All tests passed successfully.