Fix momentum begin dispatch order

Summary:
In `RCTScrollView`, it was possible that `onMomentumScrollBegin` gets queued after `onMomentumScrollEnd` because the event would dispatch after calling `setContentOffset`. As a result, the ScrollView JS gets stuck in an "animating" state and jest runners did not bother scrolling further.

## Changelog:
[iOS][Fixed] - Momentum begin events now queue before momentum end events on scroll views

Differential Revision: D65846878

packages/react-native/React/Views/ScrollView/RCTScrollView.m

@@ -599,10 +599,12 @@ - (void)scrollToOffset:(CGPoint)offset animated:(BOOL)animated
       y = fmin(y, CGRectGetMaxY(maxRect));
       offset = CGPointMake(x, y);
     }
-    [_scrollView setContentOffset:offset animated:animated];
+    // Notify of momentum scroll begin before setting content offset or events can fire out of order and scrollview gets
+    // stuck in "animating" state
     if (animated) {
       [self sendScrollEventWithName:@"onMomentumScrollBegin" scrollView:_scrollView userData:nil];
     }
+    [_scrollView setContentOffset:offset animated:animated];
   }
 }
 
@@ -624,10 +626,12 @@ - (void)scrollToEnd:(BOOL)animated
   if (!CGPointEqualToPoint(_scrollView.contentOffset, offset)) {
     // Ensure at least one scroll event will fire
     _allowNextScrollNoMatterWhat = YES;
-    [_scrollView setContentOffset:offset animated:animated];
+    // Notify of momentum scroll begin before setting content offset or events can fire out of order and scrollview gets
+    // stuck in "animating" state
     if (animated) {
       [self sendScrollEventWithName:@"onMomentumScrollBegin" scrollView:_scrollView userData:nil];
     }
+    [_scrollView setContentOffset:offset animated:animated];
   }
 }