Support CombinatorialMemberData for IEnumerable<string>

[siewers]

Fixes #54

[siewers]

I don't know why the build fails. I changed the pinned .NET SDK version to 6.0.401, which is the latest .NET 6 version. Otherwise, I couldn't get it to build locally. I don't think it's the reason it fails though.

I decided to remove the check and test for non-generic IEnumerable return types, as the next error message clearly explains the return type should implement the generic IEnumerable<T>.

Besides that, I added explicit checks for IEnumerable<T[]> and IEnumerable<IEnumerable<T>>. I'm not sure if it's strictly necessary to split it, but I don't think it hurts to be explicit and return a descriptive message about any expected common mistakes the user might make.

[siewers]

@AArnott I know you might be busy, but this is blocking me from using this library at the moment. Would you be able to have a look? Thanks 🙂

[AArnott]

Yes, thank you for submitting the change. I'll take a look.

[timba]

The latest stable version v1.5.25 doesn't include this fix. Any plans releasing it anytime soon?

[AArnott]

Published v1.6.24 to nuget.