feat: support Rolldown

[teemingc]

closes https://github.com/sveltejs/kit/issues/13738

This PR:

• upgrades vite-plugin-svelte to the v6 pre-release
• makes the Vite config changes to support both https://github.com/vitejs/rolldown-vite and the current Vite release with Rollup.
• drops support for node 18

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