fix: bail out if slot name changes and $$slots assigned to variable

[paoloricciuti]

Svelte 5 rewrite

Closes #13667

For the moment i've decided to bail out if we found a snippet that would change name. @dummdidumm was arguing that we should migrate dashed-slot to dashed_slot but since at the usage site we bail out if we found a non valid identifier i think we should bail inside the component too.

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