fix(types): Export WebViewMessageEvent, WebViewNavigation for external use (#855)

index.d.ts

 // eslint-disable-next-line
 import { IOSWebViewProps, AndroidWebViewProps } from './lib/WebViewTypes';
 
+export { WebViewMessageEvent, WebViewNavigation } from "./lib/WebViewTypes";
+
 export type WebViewProps = IOSWebViewProps & AndroidWebViewProps;
 
 declare class WebView extends Component<WebViewProps> {