Add netstandard20 target

[Hona]

Closes #654

Welcome!

Thanks for your interest in contributing to this project. Any contribution will
be gladly accepted, provided that they are generally useful and follow the
conventions of the project.

1. Please create one pull request for each feature. This results in smaller pull requests that are easier to review and validate.

2. Avoid reformatting existing code unless you are making other changes to it.

   • Cleaning-up of usings is acceptable, if you made other changes to that file.
   • If you believe that some code is badly formatted and needs fixing, isolate that change in a separate pull request.

3. Always add one or more unit tests that prove that the feature / fix you are submitting is working correctly.

4. Please describe the motivation behind the pull request. Explain what was the problem / requirement. Unless the implementation is self-explanatory, also describe the solution.

   • Of course, there's no need to be too verbose. Usually one or two lines will be enough.

5. Follow the project's coding conventions

[EdwardCooke]

This was done in another PR. #693