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