Support Svelte 3

fixes #546, #551, #552, #554

test/apps/credentials/src/routes/credentials/index.html

@@ -1,11 +1,13 @@
-<h1>{message}</h1>
+<script context="module">
+	export function preload({ query }) {
+		return this.fetch(`credentials/test.json`, {
+			credentials: query.creds
+		}).then(r => r.json());
+	}
+</script>
 
 <script>
-	export default {
-		preload({ query }) {
-			return this.fetch(`credentials/test.json`, {
-				credentials: query.creds
-			}).then(r => r.json());
-		}
-	};
-</script>
+	export let message;
+</script>
+
+<h1>{message}</h1>