Add deliberate crash just to demo recovery

[lgmfred]

:player2's token must land on row 4, col 6 to simulate a crash as below

        col 0  1  2  3  4  5  6
row 0       .  .  .  .  .  .  .
row 1       .  .  .  .  .  .  .
row 2       .  .  .  .  .  .  .
row 3       .  .  .  .  .  .  .
row 4       .  .  .  .  .  .  X  <- player2 must land here
row 5       .  .  .  .  .  .  O  <- must already be occupied

This sequence of function calls in elixir shell, for example, would produce this crash and you see the game server restarted with a new PID;

iex(1)> game_id = "crash-demo-2"
iex(2)> {:ok, pid} = ConnectFour.create_game(game_id, "Player 1")
iex(3)> :ok = ConnectFour.join_game(game_id, "Player 2")
iex(4)> :no_win = ConnectFour.drop_token(game_id, :player1, 0)
iex(5)> :no_win = ConnectFour.drop_token(game_id, :player2, 1)
iex(6)> :no_win = ConnectFour.drop_token(game_id, :player1, 6)
iex(7)> ConnectFour.drop_token(game_id, :player2, 6)