Add Window::is_maximized method (#1804)

src/platform_impl/web/window.rs

@@ -232,6 +232,12 @@ impl Window {
         // Intentionally a no-op, as canvases cannot be 'maximized'
     }
 
+    #[inline]
+    pub fn is_maximized(&self) -> bool {
+        // Canvas cannot be 'maximized'
+        false
+    }
+
     #[inline]
     pub fn fullscreen(&self) -> Option<Fullscreen> {
         if self.canvas.borrow().is_fullscreen() {