Update SvelteKit docs to accommodate SvelteKit 1.0.0-next.359 breaking changes

[metonym]

https://github.com/sveltejs/kit/blob/730bd3e831e8f98af55fe34ecf4c718c4b8663e1/packages/kit/CHANGELOG.md#100-next359

For SvelteKit >=1.0.0-next.359, the user-facing breaking change is that optimizeDeps should be defined in vite.config.js, not svelte.config.js#vite.