feat: forking

[Rich-Harris]

WIP. counterpart to sveltejs/svelte#17004

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[Rich-Harris]

Alright, I think this is ready. No tests because I'm under time pressure... and in any case we will likely want to overhaul the implementation for SvelteKit 3, as things will look different (and more fine-grained) in future. For example we will be able to maintain multiple forks simultaneously, rather than being limited to one as we are today.

But I've tested locally and it works beautifully

[paoloricciuti]

This is waiting for promise to assign to load_cache.fork but in promise we could call discard_load_cache that should clear load_cache.fork in case of an error. Wouldn't this leak fork in case of an error?

[Rich-Harris]

ooh great catch! pushed a fix