[Feature]: Allow suppressing of framework references in generated nuspec files

NuGet Product(s) Involved

NuGet SDK

The Elevator Pitch

The SuppressDependenciesWhenPacking property already allows conveniently suppressing all package dependencies. But it still includes framework dependencies. There is no property to suppress framework dependencies.

I think it makes the most sense to suppress framework dependencies as well when this same property is set. But if there are backward compat concerns for some reason, please give us another msbuild property that will suppress these.

Additional Context and Details

NuGet itself finds this useful, and has a hack to accomplish it: https://github.com/NuGet/NuGet.Client/blob/2380e11075e50104862c499515a0d3e293a7774a/src/NuGet.Clients/NuGet.CommandLine/NuGet.CommandLine.csproj#L146

I myself have hacked it out in a few other repos as well.