fix: streamed promise not resolving

[PatrickG]

Fixes #14496 and probably #14713

When a fast and a slower load function both return a promise, the count in create_async_iterator could have reached 0 before the slower load function returns. Which means the promise of the slower load function is .add()ed after the done-promise resolves.
This makes sure the done-promise only resolves after the iterator has been accessed. At that point all .add() calls have been made. This gets rid of the count var in favor of a simpler implementation.

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[Rich-Harris]

neat!