Update Babylon Native and Babylon.js to the latest

[ryantrem]

• Updating Babylon Native to the latest to pull in the iOS XR improvements (less swimmy).
• Updating Babylon.js to the latest (4.2.0-beta.1) just to stay up to date and make sure we don't have any regressions.
• Updating the README.md to remove the old note about needing C++ tooling (now all (most) of the C++ is precompiled).
• Fixing a (potential?) bug in metro.config.js (I shared our solution with the community and someone found a bug in it that didn't repro for me, but their solution seems more correct according to docs and also works for us, so I changed it to be sure).