Make event handlers functional

game.ex

@spec handle_event(String.t(), map(), Socket.t()) :: {:noreply, Socket.t()} | {:stop, Socket.t()}
def handle_event("keydown", %{"code" => code}, socket) do
  {:noreply, on_input(socket, code)}
end

def handle_event("keyup", %{"code" => code}, socket) do
  {:noreply, on_stop_input(socket, code)}
end