add string demo

c_os/user/demo/StringDemo.h

@@ -0,0 +1,16 @@
+#ifndef __StringDemo_include__
+#define __StringDemo_include__
+
+#include "kernel/Globals.h"
+
+class StringDemo : public Thread {
+private:
+    StringDemo(const StringDemo& copy) = delete;
+
+public:
+    StringDemo() : Thread("StringDemo") {}
+
+    void run() override;
+};
+
+#endif