Convert project to CPS

[sharwell]

🚧 This is a work in progress with @drewnoakes

Do not merge

[jaddie]

It would be good if not done already, to flesh out some description of what is required and perhaps even some todo items which then would be more "accessible" for others to contribute towards if they so wish etc :)

[RussKie]

My take on this is that we make the project netstandard2.0 compatible and then ensure the installer either checks for netstandard2.0 assemblies installed on a user's computer, or we bundle all necessary assemblies into the installer.

I think we already bundle up a fair share of those, @vbjay created an helper app to trace and include necessary assemblies - https://github.com/gitextensions/ReleaseFileManager

Once we're referencing netstandard2.0 the rest should be simple...

@sharwell @drewnoakes thoughts?

[drewnoakes]

This PR is only really about migrating the project from the legacy Visual Studio project system (csproj.dll) to the newer project system (based on CPS) that supports newer features such as multi-targeting.