S2 messages importable

[Flix6x]

Closes #77.

Note that the tests for importing DDBC and PEBC messages are skipped. This is due to a KeyError, which I consider out of scope for this PR. For example:

    abnormal_conditions: bool = GenPEBCInstruction.model_fields["abnormal_conditions"]  # type: ignore[assignment]
                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'abnormal_conditions'

[VladIftime]

Looks good, thanks for fixing it up.

I will fix the DDBC and PEBC messages in a separate PR.

[Flix6x]

Follow-up in #98, where we type some of the S2MessageComponents as S2MessageElements rather than S2Messages.