Make Poll an async iterable

[afshin]

In this PR, the IPoll interface and the Poll class extend AsyncIterable (by implementing a Symbol.asyncIterator method) to support for-await...of loops.

This PR also adds a README.md file for the @lumino/polling package.

This functionality may be a natural fit for streams of events from the new Jupyter event system, other WebSocket connections, or other stream sources.

cc: @Zsailer @andrii-i @davidbrochart @3coins @vidartf (since you all have some interest in events and/or iterators)

---

Example usage

The tests in this PR are illustrative. Here is one of them with some annotation:

Here, we set up the testing state variables and create a new Poll instance.

const total = 2;
let i = 0;
poll = new Poll({
  auto: false,
  factory: () => Promise.resolve(++i > total ? poll.dispose() : void 0),
  frequency: { interval: Poll.IMMEDIATE },
  name: '@lumino/polling:Poll#[Symbol.asyncIterator]-3'
});
const expected = `started ${'resolved '.repeat(total)}disposed`;
const ticker: IPoll.Phase<any>[] = [];

Then the poll is started:

void poll.start();

Instead of connecting to the ticked signal or awaiting the tick promise, we can now use a for-await...of loop:

for await (const state of poll) {
  ticker.push(state.phase);
  if (poll.isDisposed) {
    break;
  }
}

And we check to make sure the results are as expected:

// ticker and expected are both: 'started resolved resolved disposed'
expect(ticker.join(' ')).to.equal(expected);

Note for consumers of async iterators

The Poll class itself only uses ES6 (and DOM) types in its lib collection but in order to use for-await...of loops in TypeScript, you will need to use ES2018 or above in your lib array in tsconfig.json.

---

Fixes #385
cf. partially addresses #339

[blink1073]

Maybe add those notes to the README for polling?

[afshin]

I was going to ask "what README?" but the question answers itself 🤣

[blink1073]

Nice, thanks!

[fcollonval]

Thanks