Update objc2 crates (#3634)

Changes relevant to Winit:
- `icrate` has been deprecated in favour of separate crates per
  framework, in our case `objc2-foundation` and `objc2-app-kit` (and in
  the future `objc2-ui-kit` on iOS).
- Moved `MainThreadMarker::run_on_main` to free-standing function
  `run_on_main`.
- Changed how features work, this should result in less code that we
  need to compile.
- Enums are now real structs instead of type-aliases and free constants.

src/platform/ios.rs

@@ -336,7 +336,7 @@ impl MonitorHandleExtIOS for MonitorHandle {
     #[inline]
     fn ui_screen(&self) -> *mut c_void {
         // SAFETY: The marker is only used to get the pointer of the screen
-        let mtm = unsafe { icrate::Foundation::MainThreadMarker::new_unchecked() };
+        let mtm = unsafe { objc2_foundation::MainThreadMarker::new_unchecked() };
         objc2::rc::Id::as_ptr(self.inner.ui_screen(mtm)) as *mut c_void
     }