Use NAPI over JSI over JSC

[bghgary]

This change switches Babylon Native to use NAPI over JSI.

Notes:

• There are outstanding issues with the JSI implementation over JSC and thus we currently have a custom JSCRuntime.h and JSCRuntime.cpp to compensate. These changes will need to be upstreamed to Facebook in the future.
• Not all of the NAPI contract is implemented yet, but enough is implemented so that the default app runs with XR support.
• Returning NAPI types that don't match exactly with the return type (e.g. returning a Napi::Object for a function that returns Napi::Value will result in a C++ warning only when using NAPI/JSI). You must add an std::move in this situation as will be indicated by a note following the warning.
• Performance is slower with JSI due to various reasons. This will be documented in the future along with measurements.
• Code has been tested with Hermes which adds additional slowness.

TODO

• Test on iOS (@ryantrem Can you help with this?)
• Merge Babylon Native counterpart