Mark extensible enums in winit-core as #[non_exhaustive]

Event, input source, error, data transfer, native key, IME request,
scroll delta and fullscreen enums can now gain variants without a
breaking change. Backends and examples get wildcard arms where they
matched exhaustively; unknown IME requests are reported as
ImeRequestError::NotSupported and unknown fullscreen modes behave
like Fullscreen::Borderless(None).

winit-web/src/web_sys/fullscreen.rs

@@ -88,6 +88,7 @@ pub(crate) fn request_fullscreen(
                 canvas.webkit_request_fullscreen();
             }
         },
+        _ => error!("This full screen mode is not supported"),
     }
 }