Modernize NuGet packaging and release automation

[jonathanvdc]

Summary

• replace legacy .nuspec-based packaging with SDK-style dotnet pack metadata in the project files
• update GitHub Actions to version, build, test, pack, and publish NuGet packages from tags
• remove AppVeyor and the old versioning script now that GitHub Actions owns release automation
• upgrade Loyc and test-related NuGet dependencies to current versions
• fix Loyc API compatibility changes and add a small NUnit 4 compatibility shim for existing assertions

Testing

• dotnet build Pixie.sln --configuration Release --no-restore -p:Version=1.0.0-ci.2
• dotnet test Tests/Tests.csproj --configuration Release --no-build --logger "trx;LogFileName=TestResults.trx"
• dotnet pack Pixie/Pixie.csproj --configuration Release --no-build --output ./artifacts/packages -p:PackageVersion=1.0.0-ci.2
• dotnet pack Pixie.Loyc/Pixie.Loyc.csproj --configuration Release --no-build --output ./artifacts/packages -p:PackageVersion=1.0.0-ci.2