Keep the `L2CrossDomainMessenger` API backwards
compatible. The `l1CrossDomainMessenger` getter
was accidentally removed in the refactor.
This commit adds that getter back to the API.
Note that it is a legacy getter and only exists
to ensure that existing applications can still
work with the new contracts.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>