Only show retargeting dialog for .NetFramework projects (#3206)

There's several reasons for this, the most important being that only .NetFramework projects will provide a useful download. Also, not doing this resulted in a bug for Xamarin projects (Issue 2992). This fix is identical to what other projects do in Visual Studio.

vsintegration/src/FSharp.ProjectSystem.Base/Project/ProjectNode.cs

@@ -6442,10 +6442,14 @@ internal ExtensibilityEventsHelper ExtensibilityEventsHelper
 
         private bool IsTargetFrameworkInstalled()
         {
+           var targetFrameworkMoniker = new System.Runtime.Versioning.FrameworkName(GetTargetFrameworkMoniker());
+           //Only check for .NetFramework. The expectation is that other frameworks will perform any checks themselves.
+           if (targetFrameworkMoniker.Identifier != ".NetFramework")
+              return true;
+
            var multiTargeting = this.site.GetService(typeof(SVsFrameworkMultiTargeting)) as IVsFrameworkMultiTargeting;
            Array frameworks;
            Marshal.ThrowExceptionForHR(multiTargeting.GetSupportedFrameworks(out frameworks));
-           var targetFrameworkMoniker = new System.Runtime.Versioning.FrameworkName(GetTargetFrameworkMoniker());
            foreach (string fx in frameworks)
            {
                uint compat;