chore: fix $.attachment prop

[Rich-Harris]

This shouldn't be a getter. At the moment it's fine because a bug in esrap is masking it, but that bug will soon be fixed

[changeset-bot]

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Latest commit: 03bc5f4

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16172-svelte.vercel.app/

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pnpm add https://pkg.pr.new/svelte@16172

[Rich-Harris]

gonna give this the ol' cheeky self-merge to unblock some other stuff