Add deprecation warning for MaskedViewIOS (#23398)

Summary:
Add a deprecation warning for the [MaskedViewIOS](https://facebook.github.io/react-native/docs/maskedviewios) module as part of #23313.
[General] [Deprecated] - Deprecated [MaskedViewIOS](https://facebook.github.io/react-native/docs/maskedviewios) as it has now been moved to [react-native-community/masked-view](https://github.com/react-native-community/react-native-masked-view)
Pull Request resolved: #23398

Differential Revision: D14045290

Pulled By: cpojer

fbshipit-source-id: e97230f6861cf35ee0b49376d0a5c9c47d4e99f8

Libraries/react-native/react-native-implementation.js

@@ -74,6 +74,12 @@ module.exports = {
     return require('ListView');
   },
   get MaskedViewIOS() {
+    warnOnce(
+      'maskedviewios-moved',
+      'MaskedViewIOS has been extracted from react-native core and will be removed in a future release. ' +
+        "It can now be installed and imported from '@react-native-community/masked-view' instead of 'react-native'. " +
+        'See https://github.com/react-native-community/react-native-masked-view',
+    );
     return require('MaskedViewIOS');
   },
   get Modal() {