Fix MonitorHandle PartialEq and Hash on iOS

[madsmtm]

See bevyengine/bevy#16541, we were comparing the pointer addresses of MonitorHandle (which changes every time a new one is created) instead of UIScreen (which is more stable).

TODO:

• Actually test this.
  • Tested on the iOS simulator (with a single screen) and Mac Catalyst (with two monitors).
• Make a test for it.
• Add an entry to the changelog.