PowerShell Standard should list dependencies of PowerShell platforms under its facade

[rjmholt]

Today, PSStd provides an API surface that allows targeting of both Windows PowerShell and PS 6+.

However, this is only one of the functions of a facade assembly.

Another function is to help MSBuild/NuGet resolve correct dependencies for the underlying platform.

So, for example, PSStd should have a requirement on things like Newtonsoft.Json and System.Collections.Immutable, so that simply depending on PSStd will pull in those dependencies, rather than requiring an artificial entry in the csproj based on manual calculation of concrete target dependencies.

As an example of something this might fix: PowerShell/PowerShellEditorServices#1349