Set css: 'none' for SSR build

[Rich-Harris]

Closes #7555. cc @dominikg — you good with us using experimental options in this context?

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[dummdidumm]

If this doesn't provide a gracious fallback for previous Svelte versions which do not understand this option, we need to bump the peerDependency, which makes this a breaking change.

[Rich-Harris]

I'm not sure it matters anyway — this change breaks everything. We need the styles to be included in the ssr output, it turns out. Maybe we could get the styles from the csr build (but not in dev, because the csr components aren't guaranteed to be ready by the time we need them) but it's a lot of complexity for a very minor optimisation. Closing this and #7555