feat: new hook configurePreviewServer (#7658)

[benmccann]

Description

I ran git cherry-pick 20ea9999765ea372c20acc39e05cd81b98c9f6fe to pull in #7658

Additional context

We'd like to replace the svelte-kit CLI with the vite CLI and mostly use vite.config.js instead of svelte.config.js. It's a really big change both for us and our users that dwarfs the rest of the 3.x upgrade. If we can add this new feature in 2.9 as well it would allow us to start migrating users and drop a lot of brittle code that we currently have to support both code paths until this feature is available

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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