sln reader/writer (#4870)

* Snap of sln reader/writer from https://github.com/mono/monodevelop

* Remove unnecessary code

* Create a new test asset with an sln file because adding an sln to an existing one broke other tests

* Remove commented out line

* Removing unneeded PropertyGroup per PR comments

* Update the license

* Fixing the license

TestAssets/TestProjects/TestAppWithSln/Program.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace ConsoleApplication
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            Console.WriteLine("Hello World!");
+        }
+    }
+}