Move all package versions into Directory.Packages.props so every project
resolves a single consistent version. Consolidates the Roslyn packages
(Microsoft.CodeAnalysis.CSharp.Scripting/Workspaces) onto 5.0.0, which
resolves the pre-existing NU1608 version-skew error in the test projects.
17 source projects (Commons + Host + 15 components) and 17 xUnit test projects.
SLNX format, net10.0, nullable enabled, warnings as errors. All components
reference Commons; Host references all components. Builds and tests clean.