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* Create a 2-player game of Connect Four
* window.connectFour.checkForWinner(board): checks for a winner and returns null/'draw', or the winning game token.
* `board` must be a 2D array of `null` for empty spaces, and primitive values for game tokens (e.g. numbers/strings: 1 or 2, 'red' or 'yellow', 'player 1' or 'player 2')