fix: correctly prune key/each blocks

[Rich-Harris]

fixes part of #14399, and makes the code a bit less chaotic

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[changeset-bot]

🦋 Changeset detected

Latest commit: 8a5e6dc

The changes in this PR will be included in the next version bump.

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svelte	Patch

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Playground

pnpm add https://pkg.pr.new/svelte@14403

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14403-svelte.vercel.app/

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