feat: add Window::is_visible (#2169)

* feat: add `Window::is_visible`

* use `Option<bool>`

* update doc

* move it right after `set_visible`

src/platform_impl/web/window.rs

@@ -89,6 +89,11 @@ impl Window {
         // Intentionally a no-op
     }
 
+    #[inline]
+    pub fn is_visible(&self) -> Option<bool> {
+        None
+    }
+
     pub fn request_redraw(&self) {
         (self.register_redraw_request)();
     }