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.

flake.lock

@@ -0,0 +1,26 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1720768451,
+        "narHash": "sha256-EYekUHJE2gxeo2pM/zM9Wlqw1Uw2XTJXOSAO79ksc4Y=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "7e7c39ea35c5cdd002cd4588b03a3fb9ece6fad9",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-unstable",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}