Refactor to generating `S2Message` class

[Flix6x]

          We already have a version of this file in `generate_s2_message_type_to_class.py` to generate a mapping from `message_type` literals into the appropriate class. You are using it to generate a list of all message types into an alternative `S2Message` type which only contains messages.

I think we can combine the 2. My proposal would be to move TYPE_TO_MESSAGE_CLASS to the new src/s2python/message.py file and generate this entire file with a single script. The result would be that we generate both the TYPE_TO_MESSAGE_CLASS constant as well as the S2Message type.

Originally posted by @lfse-slafleur in #37 (comment)