gpui: Implement HasWindowHandle on Window (#24327)

Implement `raw_window_handle::HasWindowHandle` for `gpui::Window`

This opens a lot of possibility of using gpui with platform specific
APIs.

Edit: With this exposed, we can use crates like `window-vibrancy`,
`muda` (menus crate) or even use `wry` (a webview renderer) to create a
child `WebView` inside the gpui window.

Release Notes:

- N/A

crates/gpui/src/window.rs

@@ -23,6 +23,7 @@ use futures::FutureExt;
 #[cfg(target_os = "macos")]
 use media::core_video::CVImageBuffer;
 use parking_lot::RwLock;
+use raw_window_handle::{HandleError, HasWindowHandle};
 use refineable::Refineable;
 use slotmap::SlotMap;
 use smallvec::SmallVec;
@@ -3943,6 +3944,12 @@ impl AnyWindowHandle {
     }
 }
 
+impl HasWindowHandle for Window {
+    fn window_handle(&self) -> Result<raw_window_handle::WindowHandle<'_>, HandleError> {
+        self.platform_window.window_handle()
+    }
+}
+
 /// An identifier for an [`Element`](crate::Element).
 ///
 /// Can be constructed with a string, a number, or both, as well