Add Merge Sort Linked List algorithm

[Hardvan]

The Merge Sort for Linked List algorithm sorts a linked list by recursively dividing it into two halves, sorting each half, and then merging the sorted halves back together. It uses the divide-and-conquer approach and the merge step to ensure that the resulting linked list is sorted. The key steps include finding the middle of the list to divide it and merging two sorted lists.

Algorithm Steps:

1. Base Case: If the list is empty or contains a single node, it is already sorted.
2. Dividing the List: Use the slow and fast pointer technique to find the middle of the linked list. Split the list into two halves.
3. Recursive Sorting: Recursively apply merge sort to both the left and right halves of the list.
4. Merge Step: Merge the two sorted halves into a single sorted linked list using a helper function that compares the nodes of both lists and links them in order.
5. Result: The sorted linked list is returned after all recursive calls are complete and all sublists have been merged.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
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[cclauss]

Nice work @Hardvan

Please see some useful changes in my last commit:

• I used Python's ordered dataclasses to define the Node, which enables direct comparison on line 107.
• I used an iter function (you will need one anyway) to replace the complexities of printing and showing/testing the unsorted lists before sorting them.