Cxx TurboModules > Add example to return a JS function from Cxx to JS (#41385)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41385

Changelog: Internal

Adding a Cxx TM example which adds a listener and returns a subscription to remove that listener from the TM.

You should be able to use this with React Hooks - https://legacy.reactjs.org/docs/hooks-reference.html

E.g.

```
useEffect(() => {
  const subscription =  NativeCxxModuleExample.setValueCallbackWithSubscription(
          callbackValue => // use it
        );
  return subscription;
});
```

Reviewed By: shwanton

Differential Revision: D50473063

fbshipit-source-id: 4e9b92aeccff1771eb4ffad6bdaa20ba7f18435f

packages/rn-tester/js/examples/TurboModule/NativeCxxModuleExampleExample.js

@@ -77,6 +77,16 @@ class NativeCxxModuleExampleExample extends React.Component<{||}, State> {
       NativeCxxModuleExample?.getValueWithCallback(callbackValue =>
         this._setResult('callback', callbackValue),
       ),
+    callbackWithSubscription: () => {
+      const subscription =
+        NativeCxxModuleExample?.setValueCallbackWithSubscription(
+          callbackValue =>
+            this._setResult('callbackWithSubscription', callbackValue),
+        );
+      if (subscription) {
+        subscription();
+      }
+    },
     getArray: () =>
       NativeCxxModuleExample?.getArray([
         {a: 1, b: 'foo'},