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

build.rs

@@ -10,10 +10,9 @@ fn main() {
         android_platform: { target_os = "android" },
         web_platform: { all(target_family = "wasm", target_os = "unknown") },
         macos_platform: { target_os = "macos" },
-        ios_platform: { target_os = "ios" },
+        ios_platform: { all(target_vendor = "apple", not(target_os = "macos")) },
         windows_platform: { target_os = "windows" },
-        apple: { any(target_os = "ios", target_os = "macos") },
-        free_unix: { all(unix, not(apple), not(android_platform), not(target_os = "emscripten")) },
+        free_unix: { all(unix, not(target_vendor = "apple"), not(android_platform), not(target_os = "emscripten")) },
         redox: { target_os = "redox" },
 
         // Native displays.