chore: add visual test file to boilerplate code

tools/generate-component/boilerplate/component.visual.spec.ts

@@ -0,0 +1,23 @@
+import { html } from 'lit';
+
+import { describeViewports, visualDiffStandardStates } from '../core/testing/private.js';
+
+import './__noPrefixName__.js';
+
+describe('__name__', () => {
+  /**
+   * Add the `viewports` param to test only specific viewport;
+   * add the `viewportHeight` param to set a fixed height for the browser.
+   */
+  describeViewports(() => {
+    // Add visual tests
+    for (const state of visualDiffStandardStates) {
+      it(
+        `${state.name}`,
+        state.with(async (setup) => {
+          await setup.withFixture(html`<__name__></__name__>`);
+        }),
+      );
+    }
+  });
+});