feat: add broadcaster type inference to constructor

[Willem-Jaap]

By explicitly intersecting the EchoOptions<T> type with { broadcaster: T }, we make the broadcaster field the source of truth for generic parameter T.

Previously, when only using EchoOptions<T> TypeScript couldn’t reliably infer T from the value of options.broadcaster.
This meant you either had to manually specify the type parameter (new Echo<"pusher">({...})) or you’d lose precise typing for the connector and channel methods.

// ✅ Type is automatically inferred from broadcaster
const echo1 = new Echo({
    broadcaster: "pusher",
    key: "my-key"
});
// Type: Echo<"pusher">
// echo1.connector is PusherConnector<"pusher">

const echo2 = new Echo({
    broadcaster: "socket.io"
});
// Type: Echo<"socket.io">
// echo2.connector is SocketIoConnector

// Channel methods are also correctly typed (this would previously be `any`):
const channel1 = echo1.channel("my-channel");
// Type: PusherChannel<"pusher">

const channel2 = echo2.channel("my-channel");
// Type: SocketIoChannel

[joetannenbaum]

Good call, thank you!