[fix] prerendering always false in global code of hooks.js #4265

[Theo-Steiner]

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This fixes the behavior documented in #4265, where prerendering imported from '$app/env' is false outside the actual hooks in hooks.ts even when called during prerender.
This occurs because the hooks module is imported by the same module that exports the override function that allows to set prerendering in the first place. Therefore, when the global code of the hooks module is executed as it is imported, prerendering can never be any other value than it's default value.

This PR addresses this behavior by extracting the setting of prerendering into its own module (notify_prerendering.js), which can be imported without triggering the global code of the hooks module. As the notify_prerendering.js module will never be used anywhere other than during prerendering, it is not included into the writeServer() output of the builder.

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Alternatives considered

Instead of making these changes, this behavior could also be documented under $app/prerendering as people can circumvent the problem with a setup function as described in #4265.

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Latest commit: 8f9ff86

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[Rich-Harris]

Thank you. There's actually a simpler fix, which is more in line with plans for future changes (namely the addition of scoped __middleware.js files, which would likely take the place of hooks.js) — we import the hooks lazily rather than eagerly. I've updated the branch