Adds basic drop_tile implementation. Though doesn't check for wins
Adds an interactive mode for testing drop_tile, which allows the player
to input which tile-space to drop on.
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.
