Warn on exhaustive exported enums

Enable clippy::exhaustive_enums in every crate so new exported enums
are either #[non_exhaustive] or explicitly allowed as a closed set.

winit-uikit/src/lib.rs

@@ -100,6 +100,8 @@
 //! [app-delegate]: https://developer.apple.com/documentation/uikit/uiapplicationdelegate?language=objc
 #![cfg(target_vendor = "apple")] // TODO: Remove once `objc2` allows compiling on all platforms
 
+#![warn(clippy::exhaustive_enums)]
+
 mod app_state;
 mod event_loop;
 mod monitor;