Add snapshots with new arch only. (#56888)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56888

Adds new variant of snapshots called `newarch` with the `RCT_REMOVE_LEGACY_ARCH` flag set to `true`. This covers changes related to moving legacy code behind the ifndef, causing modifications to the API surface when the flag is enabled.

Changelog:
[Internal]

Reviewed By: cipolleschi

Differential Revision: D105686175

fbshipit-source-id: 9e24fe728ee386fb3ee7f5398fa3c55c39a8df0d