Use github workflows for CI

[piksel]

To enable better feedback and integration, this PR adds Github Actions to build and test merges into master and pull requests.
This includes running on windows, macOS and linux platforms and both on .NET Framework (on windows) and .NET Core.

Fixes #434

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.

[Numpsy]

Not directly a question about the CI system, but related and something I've wondered before:

What, at this point, does the manual TestBootstrapper app give you as compared to just using the

dotnet test

command on the test project?
The test command should be operable cross platform, and handle multi-targeted tests and such, which seems useful for the test builds?

[piksel]

If I recall correctly, it's the report. But whatever reason there used to be, it might not be valid anymore.

[Numpsy]

I believe there is an appveyor logger for dotnet test, and looking around I see https://github.com/Tyrrrz/GitHubActionsTestLogger, which sounds like it might be useful for displaying test failures in the Actions output?

[Numpsy]

The actions documentation suggests that there is a macOS runner available for builds - might be use to test with that on top on Windows and Linux?

[Numpsy]

is the 'Build and Test PR' script running the tests as part of the 'build' task, and then again as part of 'test' task?

[Numpsy]

Is 2.2.108 the appropriate version to use at this point? (I mean, I think it's one of the versions that's out of support, and I think the other PR for the nullability changes would require a v3 SDK for C#8 support)

[piksel]

Yeah, I think using the latest stable would be more appropriate