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

tests/YGStyleAccessorsTest.cpp

@@ -76,8 +76,7 @@
     INDEX_ACCESSOR_TESTS(__VA_ARGS__)(NAME, IDX, __VA_ARGS__) \
   }
 
-namespace facebook {
-namespace yoga {
+namespace facebook::yoga {
 
 using CompactValue = detail::CompactValue;
 
@@ -255,5 +254,4 @@ ACCESSOR_TEST(
 
 #endif
 
-} // namespace yoga
-} // namespace facebook
+} // namespace facebook::yoga