Condition package properties on IsPackable property

A project can only be packed if the `IsPackable` property is true.

For example:

1. Create a new ASP.NET Core Web Application
2. Right click the project and select Pack
3. See warning:
> C:\Program Files\dotnet\sdk\6.0.300-preview.22178.8\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(203,6): warning : **This project cannot be packaged because packaging has been disabled. Add true to the project file to enable producing a package from this project.**

Note that the project can be made packable by manually setting the `IsPackable` property to `true`.

Even though this web app is not packable, we still show all the package properties in the UI.

I can see two possible ways to improve this:

1. Model `IsPackable` as a non-visible property in `ConfigurationGeneralPage.xaml`
2. Model `IsPackable` as a visible and editable property in `PackagePropertyPage.xaml`

In either case, we can use visibility conditions to hide pack-related properties from the UI. Note that some properties on the "Packaging" page, such as versions, have utility outside of pack actions. We would need to do a proper review here.