rename .html files to .svelte

test/apps/session/src/routes/session.svelte

@@ -0,0 +1,10 @@
+<script>
+	import { getSession } from '@sapper/app';
+	const session = getSession();
+</script>
+
+<h1>{$session.title}</h1>
+
+<button on:click="{() => session.set({ title: 'changed' })}">
+	click me
+</button>