LibWeb: Support window.screen{X,Y,Left,Top}

Some sites query these properties for whatever reason, so let's support
them. (But let's always pretend the screen coordinates are (0, 0))

Userland/Libraries/LibWeb/DOM/Window.h

@@ -89,6 +89,9 @@ public:
 
     float device_pixel_ratio() const;
 
+    int screen_x() const;
+    int screen_y() const;
+
 private:
     explicit Window(Document&);