Add @babel/preset-env and an example test

package.json

@@ -31,6 +31,7 @@
     "svelte-jester": "^1.7.0"
   },
   "dependencies": {
+    "@babel/preset-env": "^7.14.8",
     "sirv-cli": "^1.0.0"
   },
   "jest": {

src/App.test.js

@@ -0,0 +1,15 @@
+/**
+ * @jest-environment jsdom
+ */
+
+import "@testing-library/jest-dom/extend-expect";
+
+import { render } from "@testing-library/svelte";
+
+import Comp from "./App.svelte";
+
+test("shows proper heading when rendered", () => {
+  const { getByText } = render(Comp, { name: "World" });
+
+  expect(getByText("Hello World!")).toBeInTheDocument();
+});