Support Svelte 3

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

runtime/src/app/index.ts

@@ -0,0 +1,12 @@
+import { getContext } from 'svelte';
+import { CONTEXT_KEY, stores } from '@sapper/internal/shared';
+
+export const preloading = { subscribe: stores.preloading.subscribe };
+export const page = { subscribe: stores.page.subscribe };
+
+export const getSession = () => getContext(CONTEXT_KEY);
+
+export { default as start } from './start/index';
+export { default as goto } from './goto/index';
+export { default as prefetch } from './prefetch/index';
+export { default as prefetchRoutes } from './prefetchRoutes/index';