docs: update SvelteKit/Vite usage

[metonym]

As of SvelteKit version 1.0.0-next.359, vite options are defined in vite.config.js.

import { sveltekit } from "@sveltejs/kit/vite";

/** @type {import('vite').UserConfig} */
const config = {
  plugins: [sveltekit()],
  optimizeDeps: {
    include: ["highlight.js", "highlight.js/lib/core"],
  },
};

export default config;

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