fix: flushPending in async write

[dai-shi]

We used to have isSync in writeAtomState to conditionally flush pending, but when I did #2463, I removed it. I thought it worked without it, but the behavior wasn't something expected. This PR introduces isSync again in writeAtomSync, and also in mountAtom and unmountAtom.

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Name	Status	Preview	Comments	Updated (UTC)
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Size Change: +369 B (+0.41%)

Total Size: 89.7 kB

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./dist/esm/vanilla.mjs	3.78 kB	+67 B (+1.8%)
./dist/system/vanilla.development.js	3.86 kB	+64 B (+1.69%)
./dist/system/vanilla.production.js	2.03 kB	+49 B (+2.47%)
./dist/umd/vanilla.development.js	5.1 kB	+78 B (+1.55%)
./dist/umd/vanilla.production.js	2.72 kB	+33 B (+1.23%)
./dist/vanilla.js	5 kB	+78 B (+1.58%)

ℹ️ View Unchanged

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---

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[dmaskasky]

Edge case: do we care if this changes the behavior a little bit for subscriber listeners?

const a = atom(0)
const w = atom(null, (get, set) => {
  console.log('before write')
  set(a, v=> v + 1)
  console.log('after write')
})
w.onMount = (setSelf) => {
  setSelf()
  console.log('after setSelf')
}
store.sub(a, () => {
  console.log('a listener fired')
})
store.sub(w, () => {})
/*
  CURRENT:
    1. 'before write'
    2. 'a listener fired'
    3. 'after write'
    4. 'after setSelf'

  NEW:
    1. 'before write'
    2. 'after write'
    3. 'after setSelf'
    4. 'a listener fired'
*/ 

[dai-shi]

Is it why the current test is failing?

Do you know how to make a failing test if we don't have isSync?

[dmaskasky]

Is it why the current test is failing?

Yes

Do you know how to make a failing test if we don't have isSync?

Thinking on this more...
I think we need to keep this synchronous call to flushPending in set. Some third-party libraries depend on this behavior, so we need some way to call all subscribers.

[dai-shi]

Hm, I was thinking more to the opposite. "NEW" behavior seems more reasonable than "CURRENT" behavior. I'm not sure if I understand those edge cases.

[dmaskasky]

Yes, I agree. This is better.

Hmm, it looks like atomWithObservable and atomWithStorage are not failing after all. I no longer think the behavior has changed in any significant way.

[dai-shi]

most work is done by @dmaskasky. thanks!