Migration to Github Actions

[sergey-tihon]

@mchaloupka let's move our discussion from gitter to this PR.

Please review, this PR should be already in good shape

• Actions instead of my Appveyor
• main for build and test (master and PR to master)
• new macOS target for testing (artifacts still from windows builds)
• release flow (with manual trigger) for NuGet package publishing (not tested)
• NUGET_KEY secret migrated from AppVeyor
• Test and example project migrated .NET 5 to .NET 6
• FAKE updated to latest version and FAKE dependencies also constrained by .NET 6 runtime
• new build status badge and cleanup of build dependencies

If evethiing OK we can merge and remove Appveyor webhooks from repo.