Migrating RNTester to Packages Directory (#29567)

Summary:
## Changelog

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This PR aims to migrate the RNTester App to `packages` directory. But is currently, open to inspect the CI issues and resolve the merge conflicts.

Currently done
 - Working on iOS
 - Working on Android
 - Detox Tests working on iOS

Need to work on
 - Errors generated by the CI builds

[General] [Changed] - Migrated the RNTester App to the packages directory.

Pull Request resolved: https://github.com/facebook/react-native/pull/29567

Test Plan: It runs on both ios and android for now and the detox iOS builds are working.

Reviewed By: cpojer

Differential Revision: D23034761

Pulled By: rickhanlonii

fbshipit-source-id: e04bb06e1c7ef15d340206090d1575a871b9e6f5