Support Svelte 3

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

test/apps/ignore/src/routes/b/index.html

@@ -1,11 +1,13 @@
-<h1>{letter}</h1>
+<script context="module">
+	export function preload() {
+		return this.fetch('b.json').then(r => r.json()).then(letter => {
+			return { letter };
+		});
+	}
+</script>
 
 <script>
-	export default {
-		preload() {
-			return this.fetch('b.json').then(r => r.json()).then(letter => {
-				return { letter };
-			});
-		}
-	};
-</script>
+	export let letter;
+</script>
+
+<h1>{letter}</h1>