Add the netstandard2.0 target

I'm submitting a...

Feature request

Current behavior

It works both with .NET Framework and .NET Core but I keep getting this message:

warning NU1701: Package 'Uno.SourceGeneration 3.0.0-dev.10' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8' instead of the project target framework '.NETStandard,Version=v2.0'. This package may not be fully compatible with your project.

Expected behavior

Can you add the netstandard2.0 target?

Minimal reproduction of the problem with instructions

Environment

Nuget Package: 
Uno.SourceGeneration
Package Version(s): 
3.0.0-dev.10
Affected platform(s):
- [ ] iOS
- [ ] Android
- [ ] WebAssembly
- [x ] Windows
- [ ] Build tasks

Visual Studio
- [x ] 2019 (version: )
- [ ] 2017 (version: )
- [ ] 2017 Preview (version: )
- [ ] for Mac (version: )

Relevant plugins
- [ ] Resharper (version: )