Introduce getActiveChildNavigationOptions (#5080)

This is a utility to allow navigationOptions definition functions to access the navigation options of the active child route, which allows people to replicate the v1 behavior of deep nav configuration.

src/react-navigation.js

@@ -106,6 +106,11 @@ module.exports = {
   get validateRouteConfigMap() {
     return require('./routers/validateRouteConfigMap').default;
   },
+
+  // Utils
+  get getActiveChildNavigationOptions() {
+    return require('./utils/getActiveChildNavigationOptions').default;
+  },
   get pathUtils() {
     return require('./routers/pathUtils').default;
   },

src/utils/getActiveChildNavigationOptions.js

@@ -0,0 +1,9 @@
+const getActiveChildNavigationOptions = (navigation, screenProps) => {
+  const { state, router, getChildNavigation } = navigation;
+  const activeRoute = state.routes[state.index];
+  const activeNavigation = getChildNavigation(activeRoute.key);
+  const options = router.getScreenOptions(activeNavigation, screenProps);
+  return options;
+};
+
+export default getActiveChildNavigationOptions;