C#: Add some example struct types, including one with a default constructor declarations.

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

@@ -0,0 +1,19 @@
+using System;
+
+// Struct with user declared parameterless constructor.
+public struct MyStructParameterlessConstructor
+{
+    public int X;
+    public readonly int Y;
+    public int Z { get; }
+
+    public MyStructParameterlessConstructor()
+    {
+        X = 1;
+        Y = 2;
+        Z = 3;
+    }
+}
+
+// Struct with compiler generated parameterless constructor.
+public struct MyDefaultStruct { }