dotnet add uses ProjectTypeGuid or DefaultProjectTypeGuid (#5218)

* dotnet add uses ProjectTypeGuid or DefaultProjectTypeGuid

* Create ProjectInstanceExtensions per PR comments

TestAssets/TestProjects/SlnFileWithNoProjectReferencesAndCSharpProject/CSharpProject/Program.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace CSharpProject
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+        }
+    }
+}