This is an alternative to #14121, which eliminates the (already remote) possibility of race conditions. It does so by making the entire boot payload (the __sveltekit_abc123 = {...} object that exists in the hydration <script>) accessible via a __SVELTEKIT_PAYLOAD__ define.

No test because the failure conditions are quite hard to engineer. But this general approach feels more robust than what we have now, and could enable further simplifications (by reducing the need for virtual modules, etc)

Fixes #14120

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