[transducers] feature async generators

[ja0nz]

As a big fan of async generators I would love to see some support from the @thi.ng/transducers package.
Conceptually they are not too far from normal iterators anyway so I hope this feature is possible.

	Iterators	Async iterators
Object method to provide iterator	Symbol.iterator	Symbol.asyncIterator
next() return value is	any value	Promise
to loop, use	for..of	for await..of

Example usage: NodeJS - simple directory recursion

import { opendir } from "node:fs/promises";
import { extname, join } from "node:path";

async function* walk(dir) {
  for await (const d of await opendir(dir)) {
    const entry = join(dir, d.name);
    if (d.isDirectory()) yield* walk(entry);
    else if (d.isFile()) yield entry;
  }
}

const collectMarkdownFiles = await transduce(filter(p => extname(p) === ".md"), push(), walk('.'))

Example usage: Alternating URL fetch

async function* getResources() {
  yield fetch('https://someurl.org')
  yield fetch('https://someFallback.org')
}

// Imperative usage
for await (const res of getResources()) {
  if (res.status === 404) continue;
  // response handling
  break;
}

// Declarative usage
const payload = await transduce(/* response handling */, push(), getResources())

[postspectacular]

Just a quick one: There's a 3.5 year old branch for async transducers. I stopped with it due to widespread lack of async/await support back then. I've been meaning to revisit it for a while, just no bandwidth so far. If you're interested in taking a peek:

https://github.com/thi-ng/umbrella/blob/feature/async-transducers/packages/async/src/index.ts