Add Fabric Interop constants example (facebook#36417)

Summary:
Pull Request resolved: facebook#36417

This changes adds an example to RNTester to verify that the Interop Layer can process constants in Fabric as it used to do in Paper.

## Changelog:
[iOS][Added] - Add example in the Interop Layer to use constants

Reviewed By: cortinico

Differential Revision: D43911916

fbshipit-source-id: 585669a3c997fc0f60a71789bafa7223ebff5c1c

packages/rn-tester/NativeComponentExample/ios/RNTMyLegacyNativeViewManager.mm

@@ -17,6 +17,10 @@ @interface RNTMyLegacyNativeViewManager : RCTViewManager
 
 @implementation RNTMyLegacyNativeViewManager
 
++(BOOL)requiresMainQueueSetup {
+  return NO;
+}
+
 RCT_EXPORT_MODULE()
 
 RCT_REMAP_VIEW_PROPERTY(color, backgroundColor, UIColor)
@@ -31,4 +35,10 @@ - (UIView *)view
   return view;
 }
 
+- (NSDictionary *)constantsToExport
+{
+  return @{
+    @"PI": @3.14
+  };
+}
 @end

packages/rn-tester/NativeComponentExample/js/MyNativeView.js

@@ -16,6 +16,7 @@ import RNTMyNativeView, {
 } from './MyNativeViewNativeComponent';
 import RNTMyLegacyNativeView from './MyLegacyViewNativeComponent';
 import type {MyNativeViewType} from './MyNativeViewNativeComponent';
+import {UIManager} from 'react-native';
 
 const colors = [
   '#0000FF',
@@ -76,6 +77,10 @@ export default function MyNativeView(props: {}): React.Node {
         }
       />
       <Text>HSBA: {hsba.toString()}</Text>
+      <Text>
+        Constants From Interop Layer:{' '}
+        {UIManager.RNTMyLegacyNativeView.Constants.PI}
+      </Text>
       <Button
         title="Change Background"
         onPress={() => {