appkit: Implementation of keycode_to_key

[Aceeri]

#2678

win32: #4454

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

Abusing the IME example a bit to call the winit_appkit::keycode_to_key(...):

Screen_Recording_2025-12-29_at_6.34.04_PM.mov

[NeuralModder]

Thanks for starting work on this. When I looked at the platform APIs I was concerned that this would be difficult, as I couldn't find an equivalent way to query this information from a layout with Cocoa, and Carbon is supposed to be "deprecated". But it seems winit already uses Carbon, so it seems like that isn't much of a problem.