C#: Add example of const interpolated string.

Author: michaelnebel

csharp/ql/test/library-tests/csharp10/ConstInterpolatedString.cs

@@ -0,0 +1,8 @@
+using System;
+
+public class MyConstStringInterpolationClass
+{
+    public const string hello = "Hello";
+    public const string helloWorld = $"{hello} World";
+    public const string reallyHelloWorld = $"Really {helloWorld}";
+}