fix(typings): make Manager#opts public (#1437)

As stated in the [documentation][1], the query option can be set by
setting `socket.io.opts` but `opts` was private and readonly.

[1]: https://socket.io/docs/v3/client-api/#With-query-option

lib/manager.ts

@@ -279,7 +279,7 @@ export class Manager extends Emitter {
   _reconnecting: boolean;
 
   private readonly uri: string;
-  private readonly opts: Partial<ManagerOptions>;
+  public opts: Partial<ManagerOptions>;
 
   private nsps: Record<string, Socket> = {};
   private subs: Array<ReturnType<typeof on>> = [];