Schedule work on the microtask queue

Promise callbacks and setTimeout callbacks do not share a queue. Everything queued by a resolved promise runs as soon as the current synchronous code finishes, and only after that queue is empty does the engine reach for a timer callback. Batching libraries lean on this to collapse many updates into one.

Implement soon(fn). It schedules fn to be called on the microtask queue: after the currently running synchronous code, and before any setTimeout callback, even one that was scheduled earlier with a delay of 0. Callbacks scheduled with several soon calls run in the order they were scheduled, and a soon called from inside a scheduled callback still runs before any pending timer.

What it has to do

• Never call fn synchronously.
• Call fn before any already-scheduled setTimeout callback.
• Run several scheduled callbacks in scheduling order.
• Do not use setTimeout to schedule the callback.

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