Fixes #5034 - change private property io.httpServer to a public property

[tnfAngel]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behavior

import { Server } from "socket.io";

const io = new Server(3000, { /* options */ });

io.httpServer // Property 'httpServer' is private and only accessible within class 'Server<ListenEvents, EmitEvents, ServerSideEvents, SocketData>'.ts(2341)

New behavior

import { Server } from "socket.io";

const io = new Server(3000, { /* options */ });

io.httpServer // now is accessible as the documentations says (https://socket.io/docs/v4/server-initialization/#standalone)

Other information (e.g. related issues)

#5034

[darrachequesne]

Good catch, thanks 👍