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.

main.c

@@ -0,0 +1,10 @@
+#include "connect4.h"
+#include <stdlib.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+  struct Board *my_board = make_board(6, 7);
+  print_board(my_board);
+  return EXIT_SUCCESS;
+}