fix: error if each block has key but no as clause

[Rich-Harris]

alternative to #16963. also a drive-by formatting fix

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🦋 Changeset detected

Latest commit: 51bc88e

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Playground

pnpm add https://pkg.pr.new/svelte@16966

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16966-svelte.vercel.app/