feat(iOS WKWebView): Add mainDocumentURL to onShouldStartLoadWithRequest(#493)

* Pass mainDocumentURL to onShouldStartLoadWithRequest

* Update reference

* Update typescript types

* Update WebViewTypes.ts

docs/Reference.md

 canGoBack
 canGoForward
 lockIdentifier
+mainDocumentURL (iOS only)
 navigationType
 ```
 

ios/RNCUIWebView.m

     NSMutableDictionary<NSString *, id> *event = [self baseEvent];
     [event addEntriesFromDictionary: @{
       @"url": (request.URL).absoluteString,
+      @"mainDocumentURL": (request.mainDocumentURL).absoluteString,
       @"navigationType": navigationTypes[@(navigationType)]
     }];
     if (![self.delegate webView:self

ios/RNCWKWebView.m

     NSMutableDictionary<NSString *, id> *event = [self baseEvent];
     [event addEntriesFromDictionary: @{
       @"url": (request.URL).absoluteString,
+      @"mainDocumentURL": (request.mainDocumentURL).absoluteString,
       @"navigationType": navigationTypes[@(navigationType)]
     }];
     if (![self.delegate webView:self

src/WebViewTypes.ts

     | 'reload'
     | 'formresubmit'
     | 'other';
+  mainDocumentURL?: string;
 }
 
 export type DecelerationRateConstant = 'normal' | 'fast';