Refactor zone replies for simple use case and alignment with `gen_server`

[cmdrk]

Zones currently accept 3 types of replies in a handler:

• noreply: do nothing
• {'@', [ID1, ID2,...]} : send messages to a list of IDs
• broadcast : send messages to all connected players

I propose a 4th, simpler type: reply. Send messages back to the caller.

Right now both (ENet, WebSocket) support this, but it's languishing. in ow_zone for the most part, we just reply with ok.

The second thing to do is reformat the return values from these handlers. Today, the value returned from the callback functions looks like this:

handle_foo(Msg, FromID, State) -> 
  % Your code here
  %
  Reply = {'@', [ID1, ID2,...]}, ReplyMsg},
  % OR
  Reply = {broadcast, ReplyMsg},
  % OR
  Reply = noreply,
  % Then finally:
  {Reply, State}.

Let's make it more like gen_server by instead implementing the simple reply type and also adding the @ and broadcast message types, such that:

handle_foo(Msg, FromID, State) -> 
  % Your code here
  %
  {reply, Msg1, State}.
  % OR
  {broadcast, Msg1, State}.
  % OR
  {{at,  [ID1, ID2,...]}, Msg1, State}.

It occurs to me that at is shorter than '@', since @ isn't an atom by itself! 🤦