[chore] return config from server start methods

[benmccann]

We shouldn't read the Kit config in the CLI, but rather in the Vite plugin so that it will work with the vite CLI as well. This doesn't move the config reading all the way to the plugin yet, but does remove it from the CLI which is a step in that direction

[changeset-bot]

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Latest commit: acebe26

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[bluwy]

We shouldn't read the Kit config in the CLI, but rather in the Vite plugin so that it will work with the vite CLI as well

If it's moved into a Vite plugin, then we would have to undo #4760 since we can't pass the config between Vite plugins, unless using Rollup's api convention (which isn't the most ergonomic API). We'd be loading the config twice, but it isn't too bad.

Another point from vite-plugin-svelte experience though, is that loading the Svelte config in the Vite plugin is a PITA. Due to it's async nature, it's hard to split up into smaller segmentized Vite plugins if needed in the future.

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The PR now is still great though.