Develop a fully responsive navigation menu using HTML, CSS, and JavaScript. The menu should follow a mobile-first approach, ensuring it works flawlessly on all screen sizes including mobile, tablet, and desktop. In the mobile view, display a hamburger icon in the top-right corner. When the icon is clicked, the menu should appear with a smooth sliding animation—either sliding down or from the side. It should also close when a menu item is clicked or when the user clicks outside the menu area. If there are submenus, they should be collapsible, expanding or collapsing with a “+” icon.