feat: support ?inline query on svelte style virtual modules

[bluwy]

In Vite 6, importing CSS in SSR now no longer return a default export of the transformed CSS file, you'd need to use ?inline instead. This was already pushed for in Vite 5 but we forgot to remove support in SSR.

That means frameworks who do CSS crawling to prevent FOUC will now have to use ?inline to get the transformed CSS, which most already did. However, there's a quirk where v-p-s always strips out the ?inline here:

vite-plugin-svelte/packages/vite-plugin-svelte/src/index.js

Lines 127 to 136 in da54670

	if (svelteRequest.query.type === 'style' && !svelteRequest.raw) {
	// return cssId with root prefix so postcss pipeline of vite finds the directory correctly
	// see https://github.com/sveltejs/vite-plugin-svelte/issues/14
	log.debug(
	`resolveId resolved virtual css module ${svelteRequest.cssId}`,
	undefined,
	'resolve'
	);
	return svelteRequest.cssId;
	}

This was fine for Vite 5 as mentioned above, Vite still supported default exports of CSS files in SSR, but with Vite 6 it is now empty. For v-p-s to properly support ?inline, it needs to leave the query as is, similar to ?raw handling.

---

In practice, this isn't a breaking change, however due to some unintended reliance of the old behaviour, the SvelteKit tests here will fail due to HMR (added a workaround here for now). I sent a PR upstream to fix this sveltejs/kit#13007 and if we want to play it safe for now, we can merge this only in the next major.

With this merged, it should unblock Vite 6 support for CSS FOUC for both SvelteKit and Astro.