[.NET] Add netstandard2.0 as target

[bjornharrtell]

ref #8293

[bjornharrtell]

@ccorsano would you be willing to try this out in your scenario with .NET Source Generators?

[ccorsano]

@bjornharrtell I got your changes and was able to integrate the library with my source generator test project, it looks good to me.

My little experiment then fell short as my idea required file IO (to generate a bfbs schema then parse it), which, as I now learned is heavily discouraged (to not say forbidden) in source generators.

That being said, the alternative way of achieving my current goal is to write a custom MSBuild task, which has the same constraint of requiring netstandard2.0 (due to Visual Studio still running .Net Framework internally).

Overall netstandard2.0 is the last bridge to .Net Framework, so I still think it is a valuable target to keep.

[bjornharrtell]

Hmm.. a bridge to .NET Framework is something else which I'm not to keen on myself.