Consolidate the two existing kNN implementations (#8903)

* Add type hints to k_nearest_neighbours.py

* Refactor k_nearest_neighbours.py into class

* Add documentation to k_nearest_neighbours.py

* Use heap-based priority queue for k_nearest_neighbours.py

* Delete knn_sklearn.py

* updating DIRECTORY.md

* Use optional args in k_nearest_neighbours.py for demo purposes

* Fix wrong function arg in k_nearest_neighbours.py

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