feat: support server-rendering attributes of <svelte:html> blocks

[dummdidumm]

Companion to sveltejs/svelte#14397

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13065-svelte.vercel.app/

this is an automated message

[benmccann]

Perhaps we should alphabetize these bullets?

[Conduitry]

Has it been considered to make it a build-time error rather than a warning if someone tries to use <svelte:html> without the appropriate placeholder in their app.html? That feels safer to me. I can't imagine a good reason why someone would want to have these ignored.

I don't think doing that would be a breaking change. A given component starting to use <svelte:html> (or any other new Svelte feature) is what should be considered the breaking change. Solely upgrading SvelteKit wouldn't cause a problem.

[Rich-Harris]

For that to work we'd need to read the source code of every imported component, even the ones in node_modules. We definitely could do that though it feels a bit on the grubby side

[Conduitry]

Oh, right. How about a runtime error then, rather than the runtime warning this is emitting? I still think this is safer, and I still think it shouldn't be considered a breaking change in Kit - because the real breaking change is a library starting to use this new feature and bumping its Svelte peerdep.

[Rich-Harris]

Per sveltejs/svelte#14397 (comment), I'm actually of the view that we shouldn't add this in any case (sorry)