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.

Makefile

@@ -0,0 +1,14 @@
+CC = gcc
+CFLAGS = -g -Wall -pedantic
+
+connect4.c main.c: connect4.h
+
+TARGET = connect4
+CFILES = main.c connect4.c
+OFILES = ${CFILES:.c=.o}
+
+all: ${OFILES}
+	$(CC) $(CFLAGS) -o ${TARGET} ${OFILES}
+
+clean:
+	rm -r ${TARGET} ${OFILES}