refactor(typings): export Event type (#4215)

So that it can be used by the end users:

```ts
const myMiddleware = ([eventName, ...args]: Event, next: (err?: Error) => void) => {
  console.log(eventName); // inferred as string
  next();
}

io.on("connection", (socket) => {
  socket.use(myMiddleware);
});
```

Author: shayanYousefi

lib/index.ts

@@ -830,3 +830,4 @@ module.exports.Namespace = Namespace;
 module.exports.Socket = Socket;
 
 export { Socket, ServerOptions, Namespace, BroadcastOperator, RemoteSocket };
+export { Event } from "./socket";

lib/socket.ts

@@ -107,7 +107,7 @@ export interface Handshake {
   auth: { [key: string]: any };
 }
 
-type Event = [eventName: string, ...args: any[]];
+export type Event = [eventName: string, ...args: any[]];
 
 export class Socket<
   ListenEvents extends EventsMap = DefaultEventsMap,