Update svelte.config.js to properly handle TypeScript

TypeScript syntax inside the script tags of SvelteKit routes and components isn't properly handled without adding vitePreprocess, crashing the application.

With vitePreprocess, it works.

src/dfx/assets/project_templates/svelte/src/__frontend_name__/svelte.config.js

@@ -1,7 +1,9 @@
 import adapter from '@sveltejs/adapter-static';
+import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
 
 /** @type {import('@sveltejs/kit').Config} */
 const config = {
+  preprocess: vitePreprocess(),
   kit: {
     // adapter-auto only supports some environments, see https://kit.svelte.dev/docs/adapter-auto for a list.
     // If your environment is not supported or you settled on a specific environment, switch out the adapter.