fix: make io.to(...) immutable

Previously, broadcasting to a given room (by calling `io.to()`) would
mutate the io instance, which could lead to surprising behaviors, like:

```js
io.to("room1");
io.to("room2").emit(...); // also sent to room1

// or with async/await
io.to("room3").emit("details", await fetchDetails()); // random behavior: maybe in room3, maybe to all clients
```

Calling `io.to()` (or any other broadcast modifier) will now return an
immutable instance.

Related:

- https://github.com/socketio/socket.io/issues/3431
- https://github.com/socketio/socket.io/issues/3444