[Rich-Harris]

Another piece of #16676. This began as 'we should probably move Config to @sveltejs/kit/vite and quickly became 'oh, we can simplify a ton of stuff that I've been meaning to get round to'.

It requires some corresponding changes on the svelte.dev side, which I have locally. We should merge this first though so we can cut a release, and I'll follow up with the changes over there

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Install the latest version of @sveltejs/kit from 4a67104:

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Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16752

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[Nic-Polumeyv]

svelte-package still reads options.config.kit?.alias / outDir (and config.kit?.files?.lib in cli.js), but its load_config() returns api.options, which is now the flattened config, so those all fall back to defaults.

The ...svelte_config.vitePlugin spread is gone and vitePlugin isn't in kit_options, so it gets forwarded to vite-plugin-svelte as an unknown inline option and ignored. Probably wants a removed() entry like kit got.

PR below fixes this