feat: apply inert to outroing elements

[dummdidumm]

That way they are invisible to assistive technology and can't be interacted with, which makes sense since the element is already "dead" and only transitioning out at this point. Strictly speaking that's a breaking change, so it's marked as such.

closes #8445

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Name	Status	Preview	Updated (UTC)
svelte-dev-2	❌ Failed (Inspect)		May 24, 2023 3:39pm