introduce support for aborting/timing out requests

when i started exploring what it would look like for esri leaflet to use this lib i was reminded that we allow developers to pass their own (optional) timeout (ref: https://github.com/Esri/esri-leaflet/pull/730)

a fetch can also be aborted using the technique below (in Edge and Firefox anyway).

const controller = new AbortController();
const signal = controller.signal;

setTimeout(() => controller.abort(), 5000);

fetch(url, { signal }).then(response => {
  return response.text();
}).then(text => {
  console.log(text);
});

https://developers.google.com/web/updates/2017/09/abortable-fetch

[jgravois]

relevant (and from our friend @jamesmilneruk) https://www.loxodrome.io/post/cancelling-requests/

[jgravois]

this would still be cool if anyone wants to try and put together a PR, but I won’t be tackling it anytime soon.