Rename "headers" directory to "templates", add .h template

Rename the "headers" directory to "templates", and rename the files
within as template.* with the appropriate file types so that editors
will pick up the correct type. Add a .h file template.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

templates/template.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+/* Copyright 1996-20xx The NASM Authors - All Rights Reserved */
+
+#ifndef FILENAME_H
+#define FILENAME_H
+
+#include "compiler.h"
+
+/* Code goes here */
+
+#endif /* FILENAME_H */