function create_css_import(route, url, manifest) {
	const { errors, layouts, leaf } = route;

	let css = '';

	for (const node of [...errors, ...layouts.map((l) => l?.[1]), leaf[1]]) {
		if (typeof node !== 'number') continue;
		const node_css = manifest._.client.css?.[node];
		for (const css_path of node_css ?? []) {
			css += `'${assets || base}/${css_path}',`;
		}
	}

	if (!css) return '';

	return `${create_client_import(/** @type {string} */ (manifest._.client.start), url)}.then(x => x.load_css([${css}]));`;
}
This function generates the client-side import for the CSS files that are
associated with the current route. Vite takes care of that when using
client-side route resolution, but for server-side resolution it does
not know about the CSS files automatically.

@param {import('types').SSRClientRoute} route
@param {URL} url
@param {import('@sveltejs/kit').SSRManifest} manifest
@returns {string}