Allow generated affixes for messages only or enums only

[alextsg]

Similar to #1033. I have a use case where I want to add a prefix to messages only but not to enums. Would it be useful to introduce additional options like messagePrefix and enumPrefix? typePrefix and typeSuffix could take precedence if both options are passed in. If you think this would add value I could try putting up a PR for this.

[stephenh]

Hi @alextsg ! I mean, at this point we're shooting for the moon on the number of opts that ts-proto supports, so why not! 🤣 Some sort of affixEnums=true/false maybe?

[alextsg]

🤣 thanks @stephenh . That would work for me but I was thinking messagePrefix/enumPrefix could be more flexible, in case other folks are looking to do the opposite and affix enums only and not messages, or have different prefixes/suffixes for messages vs enums.

[stephenh]

@alextsg ah I see, typePrefix is both, but messagePrefix / enumPrefix are the individual ones...yeah, that sgtm!