Send JS blur/focus events when switching to another NSWindow

[amgleitman]

Summary:

Consider a React Native experience occurring inside an NSPanel. On macOS, this NSPanel will keep track of its own first responder. In other words, if you click off that panel and into somewhere else (be it another NSWindow in the same app or another app altogether), the NSPanel never relinquishes its first responder, so we never send the blur or focus events to the JS side.

In this example of blur and focus events in Web, we see that the blur and focus events still take effect when switching windows. Therefore, it's reasonable to try to do the same in react-native-macos. We only send the events at the JS level; as far as the OS is concerned, we leave the first responders themselves intact.

Test Plan:

This was tested with the help of react-native-test-app. Specifically, I added a React Native experience hosted in an NSPanel and added some <Pressable> elements with onBlur and onFocus events. Upon switching windows, we do indeed see the blur and focus events coming through, and each NSWindow properly maintains its own first responder as desired.