fix(iOS): add canCoalesce method to RNSHeaderHeightChangeEvent (#1933)

## Description

It looks like when I was implementing class that is representing the
`onHeaderHeightChange` I made a mistake and I forgot / accidentally
removed `canCoalesce` method which led to displaying
`RNSHeaderHeightChangeEvent: unrecognized selector sent to instance` for
a couple of users.

This issue resolves that error by adding `canCoalesce` method to the
event.
Fixes #1932.

## Changes

- Added `canCoalesce` method to `RNSHeaderHeightChangeEvent` file

## Test code and steps to reproduce

You can test the changes by checking `Test1802.tsx` component in
FabricTestExample.

## Checklist

- [ ] Ensured that CI passes

ios/events/RNSHeaderHeightChangeEvent.mm

@@ -31,6 +31,21 @@ - (NSDictionary *)body
   return body;
 }
 
+- (BOOL)canCoalesce
+{
+  return YES;
+}
+
+- (uint16_t)coalescingKey
+{
+  return _headerHeight;
+}
+
+- (id<RCTEvent>)coalesceWithEvent:(id<RCTEvent>)newEvent
+{
+  return newEvent;
+}
+
 + (NSString *)moduleDotMethod
 {
   return @"RCTEventEmitter.receiveEvent";