Fix dotnet run double dash passing arguments

When run “dotnet run -- foo”, foo should be the argument passed to the
subject app. After replacing the original parser, dotnet-run did not
utilize the “unparsedtoken” of the parsed result.

To append unparsedtoken to RunCommand’s argument is not straight
forward. RunCommand has an “immutable constructor”, which is a good
thing, so I made update RunCommand’s argument following the immutable
pattern -- create a new object with the original field but only change
the arguments. I also made these filed private set.

TestAssets/TestProjects/MSBuildTestApp/Program.cs

@@ -9,6 +9,10 @@ namespace MSBuildTestApp
     {
         public static void Main(string[] args)
         {
+          if (args.Length > 0)
+            {
+                Console.WriteLine("echo args:"+ String.Join(";", args));
+            }
             Console.WriteLine("Hello World!");
         }
     }