Add configuration for disabling preferNativePlatform

[scottmas]

Summary

Currently, it is impossible to disable preferNativePlatform. This PR enables it

Test plan

In an application directory, create a blah.native.js file, then import it in your bundle entry point without its extension. If preferNativePlatform is enabled (or not present) in metro.config.js, the bundle should succeed. If it is disabled, the bundle should fail.

Note

I wasn't able to fully get my local environment working and some tests and flow checks were failing. I also did not update the snapshots, which will need to be refreshed.