Move iOS and macOS implementations into new apple module (#3756)

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`

* Add preliminary support for tvOS, watchOS and visionOS
* Reduce duplication in Cargo.toml when specifying dependencies

.github/CODEOWNERS

@@ -2,9 +2,10 @@
 /src/platform/android.rs            @MarijnS95
 /src/platform_impl/android          @MarijnS95
 
-# iOS
+# Apple (AppKit + UIKit)
 /src/platform/ios.rs                @madsmtm
-/src/platform_impl/ios              @madsmtm
+/src/platform/macos.rs              @madsmtm
+/src/platform_impl/apple            @madsmtm
 
 # Unix
 /src/platform_impl/linux/mod.rs     @kchibisov
@@ -17,10 +18,6 @@
 /src/platform/x11.rs                @kchibisov @notgull
 /src/platform_impl/linux/x11        @kchibisov @notgull
 
-# macOS
-/src/platform/macos.rs              @madsmtm
-/src/platform_impl/macos            @madsmtm
-
 # Web
 /src/platform/web.rs                @daxpedda
 /src/platform_impl/web              @daxpedda