Add check to see if solvable

[alex-stout]

For n-puzzles you can check to see if the puzzle is actually solvable by seeing the inversions.

It depends on a few things but here's the article: https://www.geeksforgeeks.org/check-instance-8-puzzle-solvable/

[tristanpenman]

Interesting find. I have been thinking about revisiting this project, mostly with the intention of modernising it a bit. A solvability check would be a nice addition.

[alex-stout]

Awesome project! If you come up with some tasks to help modernize, I would be happy to help a bit too.