refactor(sio): make Namespace._fns private (#5196)

Related: #5179

Author: ammut

packages/socket.io/lib/namespace.ts

@@ -161,7 +161,7 @@ export class Namespace<
     SocketData
   >;
 
-  protected _fns: Array<
+  private _fns: Array<
     (
       socket: Socket<ListenEvents, EmitEvents, ServerSideEvents, SocketData>,
       next: (err?: ExtendedError) => void,

packages/socket.io/lib/parent-namespace.ts

@@ -67,7 +67,7 @@ export class ParentNamespace<
   ): Namespace<ListenEvents, EmitEvents, ServerSideEvents, SocketData> {
     debug("creating child namespace %s", name);
     const namespace = new Namespace(this.server, name);
-    this._fns.forEach((fn) => namespace.use(fn));
+    this["_fns"].forEach((fn) => namespace.use(fn));
     this.listeners("connect").forEach((listener) =>
       namespace.on("connect", listener),
     );