Option to use `fetch` instead of `XMLHttpRequest` for polling

[matthiasgeihs]

Is your feature request related to a problem? Please describe.
Some environments don't support XMLHttpRequest, but support fetch.
(e.g., MetaMask Snaps)

Describe the solution you'd like
It would be great if there was an option for using fetch instead XMLHttpRequest.

Describe alternatives you've considered
There seems to be a way to solve this problem via a postinstall patch (as described in the article linked above), but native support would be favored.

[zhengcling]

I also have this need currently.

[darrachequesne]

We could indeed let the user overrides the list of available low-level transports. Not sure how it could work with tree-shaking though.

[darrachequesne]

You can now use fetch() instead of XMLHttpRequest:

import { Socket, Fetch, WebSocket } from "engine.io-client";

const socket = new Socket({
  transports: [Fetch, WebSocket]
});

Included in version 6.6.0.

[matthiasgeihs]

Very nice!