feat: add Window::is_resizable (#2171)

* feat: add `Window::is_resizable`

* move it right after `set_resizable`

src/platform_impl/windows/window.rs

@@ -225,6 +225,12 @@ impl Window {
         });
     }
 
+    #[inline]
+    pub fn is_resizable(&self) -> bool {
+        let window_state = self.window_state.lock();
+        window_state.window_flags.contains(WindowFlags::RESIZABLE)
+    }
+
     /// Returns the `hwnd` of this window.
     #[inline]
     pub fn hwnd(&self) -> HWND {