Babel preset: Add unstable_preserveClassPrivate to experiment with disabling private class transforms for SH (#55880)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55880

Disable `babel/plugin-transform-private-methods` and `babel/plugin-transform-private-property-in-object` when `customTransformOptions.unstable_preserveClassPrivate` is truthy. This allows us to experiment with native private class field and method support in Static Hermes.

Changelog: [Internal]

Reviewed By: vzaidman, javache

Differential Revision: D93010263