Replace XMLHttpRequest with fetch

[Juraj-Masiar]

Is your feature request related to a problem? Please describe.
Currently, socket.io can't be used in Chrome extension background script, since modern Manifest V3 extensions are using service worker as a background script (which doesn't have XMLHttpRequest, only fetch).
Example error:

reason
'xhr poll error'
description
TypeError: xhr.open is not a function
    at Request.create (polling.js:250:1)
    at new Request (polling.js:237:1)
    at Polling.request (polling.js:190:1)
    at Polling.doPoll (polling.js:215:1)
    at Polling.poll (polling.js:96:1)
    at Polling.doOpen (polling.js:56:1)
    at Polling.open (transport.js:46:1)
    at Socket.open (socket.js:170:1)
    at new Socket (socket.js:111:1)
    at Manager.open (manager.js:108:1)

Describe the solution you'd like
Replacing XMLHttpRequest with modern fetch.

Describe alternatives you've considered
I think using transports: ['websocket'], would help, but that way I can't use extraHeaders which I use for authorization.

Additional context

"socket.io-client": "^4.7.5",

[darrachequesne]

We could indeed provide another implementation for the HTTP long-polling transport, something like:

import { io } from "socket.io-client";

const socket = io({
  transports: [
    PollingWithFetch,
    WebSocket
  ]
});

Related: socketio/engine.io-client#716