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

src/platform_impl/apple/mod.rs

@@ -0,0 +1,11 @@
+//! Apple/Darwin-specific implementations
+
+#[cfg(target_os = "macos")]
+mod appkit;
+#[cfg(not(target_os = "macos"))]
+mod uikit;
+
+#[cfg(target_os = "macos")]
+pub use self::appkit::*;
+#[cfg(not(target_os = "macos"))]
+pub use self::uikit::*;