Add example that runs on Windows to the demo project.

It is interesting to step into the generated method and observe the UX.

DllImportGenerator/Demo/Program.cs

@@ -5,10 +5,19 @@
 
 namespace Demo
 {
+    partial class Kernel32
+    {
+        [GeneratedDllImport(nameof(Kernel32), EntryPoint = "QueryPerformanceCounter")]
+        public static partial int Method(ref long t);
+    }
+
     unsafe class Program
     {
         static void Main(string[] args)
         {
+            var ts = (long)0;
+            int suc = Kernel32.Method(ref ts);
+            Console.WriteLine($"{suc}: 0x{ts:x}");
         }
     }
 }