fix: add link rel=preload for exported sites

test/apps/export-webpack/src/server.js

@@ -0,0 +1,15 @@
+import sirv from 'sirv';
+import polka from 'polka';
+import * as sapper from '@sapper/server';
+
+const { PORT, NODE_ENV } = process.env;
+const dev = NODE_ENV === 'development';
+
+polka()
+	.use(
+		sirv('static', { dev }),
+		sapper.middleware()
+	)
+	.listen(PORT, err => {
+		if (err) console.log('error', err);
+	});