fix(preprocessor): return object, not just code (#33576)

Summary:
Jest 28 will error if only a string is returned from a transfomer, it needs to be an object of `{code: string, map?: object}`. Returning an object has been supported since https://github.com/facebook/jest/pull/2290, released in Jest v20.

## Changelog

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[Internal] [Fixed] - Return object from Jest preprocessor

Pull Request resolved: https://github.com/facebook/react-native/pull/33576

Test Plan: Green CI?

Reviewed By: motiz88

Differential Revision: D35426748

Pulled By: robhogan

fbshipit-source-id: 3edb79b62b2d34274be192061817d3fcd8a7ba17