Settings item template breaks the build in .NET Core applications

[JeremyKuhne]

If you add a new "Settings" item to a .NET Core project it will cause a build failure if you don't have the System.Configuration.ConfigurationManager package installed.

Error	CS1069	The type name 'ApplicationSettingsBase' could not be found in the namespace 'System.Configuration'. This type has been forwarded to assembly 'System.Configuration.ConfigurationManager, Version=4.0.3.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' Consider adding a reference to that assembly.	Core2	C:\Users\jerem\source\repos\UserSettings\Core2\Settings1.Designer.cs	16	Active

This doesn't happen in a .NET Core WinForms application as Microsoft.WindowsDesktop.App.WindowsForms framwork carries that package.

I'm not sure how things work precisely, but we:

1. Want to keep the template available for Core as it is still a supported scenario
2. Not add the package on .NET Framework apps, only Core, and ideally not when the WindowsForms framework is targeted (as it is redundant in that case)

cc: @ericstj; @maryamariyan