feat(async): Add abortable to async module.

[lambdalisue]

Hi.

The abortable is a wrapper function that makes Promise and AsyncIterable cancelable.

For example, in the case of Promise, it looks like this

import { abortable } from "https://deno.land/std/async/mod.ts";
import { delay } from "https://deno.land/std/async/mod.ts";
const p = delay(1000);
const c = new AbortController();
setTimeout(() => c.abort(), 100);
// Below throws `AbortedError` after 100 ms
await abortable(p, c.signal);

and for AsyncIterable as follows

import { abortable } from "https://deno.land/std/async/mod.ts";
import { delay } from "https://deno.land/std/async/mod.ts";
const p = async function* () {
  yield "Hello";
  await delay(1000);
  yield "World";
};
const c = new AbortController();
setTimeout(() => c.abort(), 100);
// Below throws `AbortedError` after 100 ms
// and items become `["Hello"]`
const items: string[] = [];
for await (const item of abortable(p(), c.signal)) {
  items.push(item);
}

I hope you all like it.

[CLAassistant]

All committers have signed the CLA.

[kt3k]

Thanks for your contribution! This seems a reasonable addition to std/async to me.

LGTM

[kt3k]

I wonder if it makes sense to use DOMException with AbortError instead of this custom error:

new DOMException(reason ? `Aborted: ${reason}` : "Aborted", "AbortError")

[crowlKats]

Yes i think this would be preferable imo. Ideally it would be using error causes, but DOMException doesnt support that yet (whatwg/webidl#969).

[lambdalisue]

Shall I rewrite it as DOMException with that?

[kt3k]

Shall I rewrite it as DOMException with that?

I think that's more preferable (fetch API rejects with that error, for example). Let's do that

[kt3k]

@lambdalisue LGTM!