[editorial] Shared steps between toArray and Array.fromAsync

[js-choi]

Array.fromAsync and AsyncIterator.prototype.toArray are similar, although Array.fromAsync will likely reach Stage 4 first.

Once Array.fromAsync lands in ecma262, it would be good to have it and AsyncIterator.prototype.toArray share the same steps via an abstract operation.

See tc39/proposal-array-from-async#23.