Add Horizontal and Vertical win checking

Adds horizontal and vertical win checking, as well as tests for such
features.

Adds valgrind to the flake.nix

Adds test_connect4 option to Makefile

connect4.h

@@ -22,6 +22,8 @@ struct Board {
 // Returns a board struct with height and width based on parameters
 struct Board *make_board(size_t height, size_t width);
 
+void free_board(struct Board *board);
+
 // Drops a tile, returns 0 if successful, -1 if error, and 1 if the move won
 int drop_tile(struct Board *board, size_t drop_pos);