Checkers Begins

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The very beginning of a checkers module! Includes some backend (just enough to proof-of-concept run it) and some terminal features (ditto on the "just enough").

Still needs double-jump, "king-ing", a check to prevent un-king-ed pieces from moving backwards (so maybe king-ing DOESN'T need to be added? 🤔) end-of-game winner messages, option for obligatory jumping... I'm sure I'll think of more.

This commit is mostly meant to allow @MicahWW to give input and suggest changes while this is still in the relatively early stages. I suspect the col,row format will be a contentious discussion, I do already have ideas for where an earlier switch-around for those could go, but we'll pin down the details. Other talking points include:

• player numbering (zero-indexing players in TicTacToe is fine but for Checkers this is a hill I will die on)
• the placement of jumpPiece in checkValidMove (I don't love it, open to conversation)
• the colors of pieces
• if out-of-bounds spaces should be colored differently than in-bounds spaces (that would take a bit of doing and would be good to start discussing now)
• invalidEntryMessage as basically a variable/constant

Lastly... can I do a Batman titles thing for the next two commits on this topic? So my next one would be "Checkers - Dark Knight" or something like that

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I tried to comment/document heavily to make it as clear as possible what's going on in each line. I should be free tomorrow to do a voice chat if you want a sherpa through the whole thing as well.