Add .NET Standard 2.0 support (multi-targeting)

[xperiandri]

As long as this library does not support any .NET 5 specifics I ask to add .NET Standard 2.0 support. Because the library cannot be used in UWP and Uno apps, as UWP does not support .NET 5 yet.

[lfr]

Hi @xperiandri, is there any plan that you're aware of for UWP to support .NET 5?

[xperiandri]

Unfortunately, it is impossible now. https://platform.uno/blog/recent-uwp-and-net-5-net-6-news-and-uno-platform-plans/ Microsoft does not plan to add such support so that I need to wait until the UWP features are ported to WinUI which can last for a year or two

[lfr]

Couldn't you use the older version until UWP/Uno support the new .Net? The functionality is the same, only the API is slightly different.

You can get a sense of the old API by checking an an older version of the GitHub, but essentially it's called "Blocks" instead of "Boxes" and the namespaces are FSharp.ValidationBlocks instead of FSharp.Domain.Valication.