Small refactor of how Fantom configs are formatted (#52826)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52826

Changelog: [internal]

This is just an intermediate change to simplify how we format Fantom configs, so we can extend it to use other formatting formats (e.g.: short format to use in file names).

Reviewed By: lenaic

Differential Revision: D78924771

fbshipit-source-id: 4886a800a6836dc6a66539b1df079adb6c9c52e1