Add basic end-to-end test

Per the last commit, this is to ensure automated dependency PRs don't
get merged and silently bring the app down

e2e/app.spec.js

@@ -0,0 +1,10 @@
+// @ts-check
+const { test, expect } = require('@playwright/test');
+
+test('homepage loads', async ({ page }) => {
+  await page.goto('/');
+
+  // Expect a title "to contain" a substring.
+  await expect(page).toHaveTitle("Home – dxw Prototype Library");
+  await expect(page.getByRole('heading', { name: "dxw Prototype Gallery" })).toBeVisible();
+});

package.json

@@ -2,7 +2,8 @@
   "scripts": {
     "dev": "govuk-prototype-kit dev",
     "serve": "govuk-prototype-kit serve",
-    "start": "govuk-prototype-kit start"
+    "start": "govuk-prototype-kit start",
+    "test": "npx playwright test"
   },
   "name": "govuk-prototype-kit",
   "dependencies": {