Remember ShadowNode of RawText's parent in attribute string

Summary:
When a user clicks on some text, `RCTSurfaceTouchHandler` will call into a method on `RCTParagraphComponentView`. That method (i.e: `touchEventEmitter`) would be responsible for identifying the closest ancestral `<Text/>` component to which we should dispatch the `onPress` event, given the point where the user clicked. To answer this query, we'll use a data structure called `UIAttributedString`.

This data structure represents a string, and a corresponding mapping from sequences of its characters to some arbitrary data. In this attributed string, we'll map sequences of characters to their closest ancestral `ParagraphShadowNode` or `TextShadowNode`. That way, when we get a click event on `RCTParagraphComponentView`, we can just look at the character that was clicked, and use that information to do a lookup in the attributed string to find the shadow node who's EventEmitter is responsible for processing the click event.

Reviewed By: shergin

Differential Revision: D9696904

fbshipit-source-id: a199649981ad271afa85414ce4c3f056851348be

ReactCommon/fabric/attributedstring/AttributedString.h

@@ -39,6 +39,7 @@ public:
     std::string string;
     TextAttributes textAttributes;
     SharedShadowNode shadowNode;
+    SharedShadowNode parentShadowNode;
   };
 
   using Fragments = std::vector<Fragment>;