Evaluate .net framework version for reusable parts

[curtnichols]

Consider the following:

• .NET Standard 2.0 supports many platforms, where 2.1 does not support .NET Framework (the older desktop framework).
• Microsoft appears not to be making progress on F# in store apps, which affects our plans for new tooling.

Per the .NET Implementation Support table, .NET Standard 2.0 supports the following:

Platform	Version
.NET Core	2.0
.NET Framework (desktop)	4.6.1
Mono	5.4
Xamarin.iOS	10.14
Xamarin.Mac	3.8
Xamarin.Android	8.0
UWP	10.0.16299
Unity	2018.1

Possible outcomes:

• downgrade dotnet-platform projects to .NET Standard 2.0.
• port some code from F# to C#.