Review targeting .NET Standard 2.0

[savornicesei]

https://docs.microsoft.com/en-us/dotnet/standard/net-standard
https://dotnet.microsoft.com/platform/support/policy/dotnet-core

There will be issues with Razor module, see this.
When upgrading Razor deps, all packages must have same major version, including Microsoft.Extensions since 3.x target .NET Core 3, 5.x targets .NET 5, see details here.

AB#9

[savornicesei]

There is an interesting discussion about .NET roll-forward feature in ikvm repo.

IMHO, targeting netstandard2.0 allows wyam2 to be embedded in any .NET framework 4.6.1+ app up to .NET 6.0. It would be nice to support 2.0, 3.1 and 6