fix: associate batch with boundary

[dummdidumm]

This associates the current batch with the boundary when entering pending mode. That way other async work associated to that boundary also can associate itself with that batch, even if e.g. due to flushing it is no longer the current batch.

This solves a null pointer exception that can occur when the batch is flushed before the next top level await or async derived gets a hold of the current batch, which is null then.

Fixes #16596
Fixes sveltejs/kit#14124

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Playground

pnpm add https://pkg.pr.new/svelte@16621

[dummdidumm]

Mhm there's a regression with this because e.g. in one case where the batch is switched mid-pending it would be wrong to associate a new-outdated batch the boundary. Mhhhm.

Update: Discovered a memory leak through this: batches that are superseeded are never cleaned up. They're put into the batches set but never cleared from it because their pending count never reaches 0.