C#: Add string interpolation examples to cs/useless-assignment-to-local.

Author: michaelnebel

csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.cs

@@ -474,3 +474,13 @@ public static void M()
         }
     }
 }
+
+class StringInterpolation
+{
+    void Pi()
+    {
+        float pi = 3.14159f; // GOOD
+        const int align = 6; // GOOD
+        Console.WriteLine($"Pi, {pi,align:F3}");
+    }
+}