fix(types): Make onContentProcessDidTerminate optional (#890)

src/WebViewTypes.ts

   pagingEnabled?: boolean;
   scrollEnabled?: boolean;
   useSharedProcessPool?: boolean;
-  onContentProcessDidTerminate: (event: WebViewTerminatedEvent) => void;
+  onContentProcessDidTerminate?: (event: WebViewTerminatedEvent) => void;
 }
 
 export interface IOSWebViewProps extends WebViewSharedProps {
    * Function that is invoked when the WebKit WebView content process gets terminated.
    * @platform ios
    */
-  onContentProcessDidTerminate: (event: WebViewTerminatedEvent) => void;
+  onContentProcessDidTerminate?: (event: WebViewTerminatedEvent) => void;
 }
 
 export interface AndroidWebViewProps extends WebViewSharedProps {