chore: lift "flushSync cannot be called in effects" restriction

[dummdidumm]

Since async-await was introduced into the code base a lot has changed. This proposes to lift the restriction that flushSync cannot be called in effects. No tests fail, and in general the whole logic is a lot more robust now (and, to be fair, the stack traces when debugging are confusing either way).

Closes #17131 (though I still wonder why Skeleton does that)

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Playground

pnpm add https://pkg.pr.new/svelte@17139

[Hugos68]

@dummdidumm Skeleton maintainer here, we don't actually call flushSyncor use any Svelte API's, we are build on top of the foundation of ZagJS. This is where they use flushSync: https://github.com/chakra-ui/zag/blob/main/packages/frameworks/svelte/src/bindable.svelte.ts#L43

They build framework agnostic state machines with framework adapters, it's quite complicated but maybe this helps understand the use case or even suggest an improvement.

Edit upon further inspection it looks like it's to flush all current effects before setting the value of a so called "bindable". Which is an internal reactive variable from the looks of it.

@segunadebayo (Creator of Zag) perhaps you could help explain why Zag uses flushSync here? (Or why it's ran inside in $effect for collapsible)