Implements https://github.com/vercel/next.js/issues/30997 with some minor tweaks to the design:
* The hook is moved to Client Components (e.g. `pages/_app` instead of `pages/_document`). This was a silly oversight in the original design: the hook needs to be called during server prerendering.
* `useFlushEffects` instead of `useFlushEffect` as there isn't a particularly safe way to implement the singular semantics as a Client Component hook given the current implementation of server rendering.
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Fixes#30997