fix: special-case width/height attribute during spread

[dummdidumm]

fixes #6752

@Conduitry please have a closer look at this - seems like there's some history with this stuff, and you might have reservations about introducing some kind of lookup table

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Name	Status	Preview	Comments	Updated (UTC)
svelte-dev-2	❌ Failed (Inspect)			Apr 2, 2023 3:30am