Add meta-files, board structure, and alloc function

Adds meta-files such as the Makefile and flake.lock/nix for easier
project management.

Adds board structure in connect4.h, as well as some helper functions
such as the make_board function to create a board, and the print_board
function to view the board after creation.

connect4.h

@@ -0,0 +1,31 @@
+#ifndef CONNECT4_H_
+#define CONNECT4_H_
+
+#include <stddef.h>
+
+enum Tile {
+  EMPTY = 0,
+  RED,
+  BLACK
+};
+
+struct Board {
+  size_t height;
+  size_t width;
+  size_t *tile_heights;
+  enum Tile *tilemap;
+  enum Tile next_player;
+};
+
+#define IDX(i, j, board) (board->tilemap[j*(board->height) + i])
+
+// Returns a board struct with height and width based on parameters
+struct Board *make_board(size_t height, size_t width);
+
+// 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);
+
+// Prints the supplied board
+void print_board(struct Board *board);
+
+#endif // !CONNECT4_H_