fix: mark subtree dynamic for img with loading attribute

[paoloricciuti]

Closes #14312

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🦋 Changeset detected

Latest commit: f13795a

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

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14317-svelte.vercel.app/

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14317