Add new environment variables to force CI and debug C++ (#52776)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52776

Changelog: [internal]

`FANTOM_FORCE_CI_MODE` is just an explicit way to indicate that we're on CI, so we'd run benchmarks in test mode, for example.

`FANTOM_DEBUG_CPP` is just an alias for `FANTOM_ENABLE_CPP_DEBUGGING` which is unnecessarily long.

Reviewed By: rshest

Differential Revision: D78801918

fbshipit-source-id: 8e60bdd911067c6b0b92be7e90553fd5209c9ca9