add standard markdown badges (dart-archive/native_synchronization#7)

Author: devoncarew

pkgs/native_synchronization/README.md

@@ -1,4 +1,6 @@
 [![Dart](https://github.com/dart-lang/native_synchronization/actions/workflows/dart.yaml/badge.svg)](https://github.com/dart-lang/native_synchronization/actions/workflows/dart.yaml)
+[![pub package](https://img.shields.io/pub/v/native_synchronization.svg)](https://pub.dev/packages/native_synchronization)
+[![package publisher](https://img.shields.io/pub/publisher/native_synchronization.svg)](https://pub.dev/packages/native_synchronization/publisher)
 
 This package exposes a portable interface for low-level thread
 synchronization primitives like `Mutex` and `ConditionVariable`.