Implement i18nManager.forceRTL

[kmelmon]

This was discovered when testing the RTLExample page in RNTester.

Near the top of the page there's a button that calls the i18nManager.forceRTL API, which is supposed to force RTL regardless of what locale the app is running in.

react-native-windows doesn't implement this API so this functionality doesn't work.

This issue is tracking adding an implementation for this API.

[kmelmon]

@kikisaints Note that this API isn't documented in RN documentation. We need to get this API documented.