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.

• [x] 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:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new Python files are placed inside an existing directory.
• [x] All filenames are in all lowercase characters with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.
• [x] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [x] If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".