Upgrade React Native (Playground & PackageTest) to 0.63.0

[ryantrem]

This change upgrades the Playground and PackageTest apps to React Native 0.63.0. I decided to hold off on updating the actual @babylonjs/react-native dependencies on React Native for now since we are currently relying changes local to this repo that have not made it into a React Native release yet (e.g. JSCRuntime.cpp). Most of these changes are "boiler plate," in that I just recreated the apps from scratch and pulled forward the few customizations we'd previously made. Some notable parts of this change:

• I updated the root .gitignore as some additional cleanup in this set of changes (I believe all the additions I made to .gitignore are generated files).
• I noticed that bitcode was still not being enabled on our libs (I had tested this before, so I'm not sure why it's not working now). I put a comment in the CMakeLists.txt and for now updated the readme and the build script to enable bitcode in the cmake command line.
• I updated the gulp build to validate the set of files that are being published for the package. This is to help prevent us from accidentally publishing a broken package.