[fabric] Implement flashScrollIndicators on Fabric scroll view

Summary:
This diff adds the flashScrollIndicators method to the `RCTEnhancedScrollView` used by `RCTScrollViewComponentView` in Fabric.

This method is used by the command with the same name to enable the flashing of the scroll bars from the JS side.

Test Plan:
Run Zeratul with Fabric enabled and with a custom VirtualizedList.js implementation that adds the following code to `_onContentSizeChange`:
```
this.getScrollResponder().flashScrollIndicators();
```

This should flash the scroll bars every time the content size of the scroll view changes. Meaning adding a message to a thread should flash the scroll bars using the `flashScrollIndicators` command.

 https://pxl.cl/3MVZ1

Reviewers: shawndempsey, #rn-desktop

Reviewed By: shawndempsey

Differential Revision: https://phabricator.intern.facebook.com/D51222019

packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTEnhancedScrollView.h

@@ -59,6 +59,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)setContentOffset:(CGPoint)contentOffset animated:(BOOL)animated;
 - (void)zoomToRect:(CGRect)rect animated:(BOOL)animated;
+- (void)flashScrollIndicators;
 #endif // macOS]
 
 /*

packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTEnhancedScrollView.mm

@@ -130,6 +130,11 @@ - (void)zoomToRect:(CGRect)rect animated:(BOOL)animated
 {
   [self magnifyToFitRect:rect];
 }
+
+- (void)flashScrollIndicators
+{
+  [self flashScrollers];
+}
 #endif // macOS]
 
 #if !TARGET_OS_OSX // [macOS]

packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm

@@ -690,9 +690,7 @@ - (void)handleCommand:(const NSString *)commandName args:(const NSArray *)args
 
 - (void)flashScrollIndicators
 {
-#if !TARGET_OS_OSX // [macOS]
-  [_scrollView flashScrollIndicators];
-#endif // [macOS]
+  [(RCTEnhancedScrollView *)_scrollView flashScrollIndicators]; // [macOS]
 }
 
 - (void)scrollTo:(double)x y:(double)y animated:(BOOL)animated