Add/remove solution items based on directory structure (#5197)

* WIP support solution folders for dotnet add and remove

* Add/remove solution folders based on directory hierarchy

* Fix tests

* Disable the solution building tests

* Address PR comments

* Fix a build break due to a new tool version used in the build

* Create SlnProjectExtensions and SlnProjectCollectionExtensions per PR comments

TestAssets/TestProjects/TestAppWithSlnAndCsprojInSubDir/Program.cs

@@ -0,0 +1,10 @@
+using System;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        Console.WriteLine("Hello from the main app");
+        Console.WriteLine(Lib.Library.GetMessage());
+    }
+}