RN: Support Private Methods & Properties (#39318)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39318

Updates `react-native/babel-preset` to support private methods and properties.

Changelog:
[General][Added] Added plugins for private methods and properties to `react-native/babel-preset`.

Reviewed By: asiandrummer

Differential Revision: D49029339

fbshipit-source-id: 2e0578886b22770d3f037ce184b842ee15a59c8e