Release v0.4.0 for react-native 0.62.x

This release includes an implementation of Proxy and Reflect, but it is not enabled by default. To enable it for the command line tools, pass the -Xes6-proxy flag, such as hermes -Xes6-proxy. Proxy will be enabled in a future release of React Native.

The 0.4.0 release is not compatible with RN 0.61.

This release also includes a number of other performance enhancements, bug fixes, and memory savings in the engine.