Add collection-no-register-item example (ariakit#3093)

Part of ariakit#939

-  Collection with `shouldRegisterItem` set to `false`

examples/collection-no-register-item/index.tsx

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+import "./style.css";
+import * as Ariakit from "@ariakit/react";
+
+export default function Example() {
+  const collection = Ariakit.useCollectionStore();
+  const length = collection.useState((state) => state.renderedItems.length);
+  return (
+    <Ariakit.Collection store={collection} className="collection">
+      <div>Items count: {length}</div>
+      <Ariakit.CollectionItem>🍎 Apple</Ariakit.CollectionItem>
+      <Ariakit.CollectionItem>🍇 Grape</Ariakit.CollectionItem>
+      <Ariakit.CollectionItem shouldRegisterItem={false}>
+        🍊 Orange
+      </Ariakit.CollectionItem>
+    </Ariakit.Collection>
+  );
+}

examples/collection-no-register-item/style.css

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+@import url("../collection/style.css");

examples/collection-no-register-item/test.ts

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+import { q } from "@ariakit/test";
+
+test("should not count unregistered items", async () => {
+  expect(await q.text.wait("Items count: 2")).toBeInTheDocument();
+});