C++ 17 style nested namespaces (#1326)

Summary:
X-link: https://github.com/facebook/react-native/pull/38304

Pull Request resolved: https://github.com/facebook/yoga/pull/1326

For better readability

Reviewed By: christophpurrer

Differential Revision: D47384926

fbshipit-source-id: 2f60d50a185331b3624d45d1fc45f98d504b3034

yoga/event/event.cpp

@@ -9,8 +9,7 @@
 #include <atomic>
 #include <memory>
 
-namespace facebook {
-namespace yoga {
+namespace facebook::yoga {
 
 const char* LayoutPassReasonToString(const LayoutPassReason value) {
   switch (value) {
@@ -82,5 +81,4 @@ void Event::publish(const YGNode& node, Type eventType, const Data& eventData) {
   }
 }
 
-} // namespace yoga
-} // namespace facebook
+} // namespace facebook::yoga