chore: export BottomTabBarHeightCallbackContext (#10617)

**Motivation**

Custom TabBarView required this to provide the correct height.

```tsx
import { useCallback, useContext } from 'react';
import {
  createBottomTabNavigator,
  BottomTabBarHeightCallbackContext, // this is required.
} from '@react-navigation/bottom-tabs';

const Tab = createBottomTabNavigator();

const BottomTabBar = () => {
  const reportHeight = useContext(BottomTabBarHeightCallbackContext);

  const handleLayout = useCallback(e => reportHeight(e.nativeEvent.layout.height), [reportHeight]);

  return (
    <View onLayout={handleLayout}>
      {/* ... */}
    </View>
  );
};

const App = (
  <Tab.Navigator tabBar={BottomTabBar}>
    {/* ... */}
  </Tab.Navigator>
);
```

packages/bottom-tabs/src/index.tsx

@@ -12,6 +12,7 @@ export { default as BottomTabView } from './views/BottomTabView';
 /**
  * Utilities
  */
+export { default as BottomTabBarHeightCallbackContext } from './utils/BottomTabBarHeightCallbackContext';
 export { default as BottomTabBarHeightContext } from './utils/BottomTabBarHeightContext';
 export { default as useBottomTabBarHeight } from './utils/useBottomTabBarHeight';