[ffigen] fix missing exports used in the FfiGenerator

[josxha]

This pr exports a missing class and some enums used by the FfiGenerator.

• fix [Interop EAP] Missing exports used in FfiGenerator (ffigen v20.0.0) #2765
• export the enums CommentLength, CommentStyle and EnumStyle
• export the class Version

Caution: The class Version comes from the pub_semver package. In general I'd say exporting an external class introduces some risk in case the package does not follow semantic versioning. In this case however it is a tools.dart.dev package, so in my opinion the risk is neglectable.

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