Move iOS and macOS implementations into new `apple` module

[madsmtm]

Move iOS and macOS implementations to a shared folder called apple, to allow us to reduce the code-duplication between these platforms in the future.

The folder structure is now:

• src/platform_impl/apple/
  • appkit/
  • uikit/
  • example_shared_file.rs
  • mod.rs

Part of https://github.com/rust-windowing/winit/issues/1029.

I considered calling the folder darwin, as that's the name of the underlying core OS that all of these platforms share, but the implementation here in Winit is really Apple-specific, i.e. even if Apple created a new OS different from Darwin, they would likely still port Foundation and UIKit to that, and the module name would still fit.

• [x] Tested on all platforms changed

[ActuallyHappening]

I'm a bit of a noob, is the uikit for iOS and appkit for macOS?

[madsmtm]

Yup, but UIKit is also for tvOS, watchOS, visionOS and Mac Catalyst.