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/NativeCxxModuleExample/NativeCxxModuleExample.js

@@ -73,6 +73,9 @@ export interface Spec extends TurboModule {
   +getUnion: (x: UnionFloat, y: UnionString, z: UnionObject) => string;
   +getValue: (x: number, y: string, z: ObjectStruct) => ValueStruct;
   +getValueWithCallback: (callback: (value: string) => void) => void;
+  +setValueCallbackWithSubscription: (
+    callback: (value: string) => void,
+  ) => () => void;
   +getValueWithPromise: (error: boolean) => Promise<string>;
   +getWithWithOptionalArgs: (optionalArg?: boolean) => ?boolean;
   +voidFunc: () => void;