feat: Add author_id property

[DataTriny]

Adds a new optional author_id property, useful primarily for QA.

• On Unix: the purpose of the AccessibleId property have been clarified since I wrote the adapter. These changes make it behave like GTK4.
• On macOS: I haven't really found other implementations of this property, but Apple documentation indicates that accessibilityIdentifier is intended for this.

Tested on Windows and Linux by modifying the mixed_handlers example like so:

diff --git a/platforms/winit/examples/mixed_handlers.rs b/platforms/winit/examples/mixed_handlers.rs
index 00281e2..7c1e68f 100644
--- a/platforms/winit/examples/mixed_handlers.rs
+++ b/platforms/winit/examples/mixed_handlers.rs
@@ -43,8 +43,14 @@ fn build_button(id: NodeId, name: &str) -> Node {
         BUTTON_2_ID => BUTTON_2_RECT,
         _ => unreachable!(),
     };
+    let author_id = match id {
+        BUTTON_1_ID => "button1",
+        BUTTON_2_ID => "button2",
+        _ => unreachable!(),
+    };

     let mut builder = NodeBuilder::new(Role::Button);
+    builder.set_author_id(author_id);
     builder.set_bounds(rect);
     builder.set_name(name);
     builder.add_action(Action::Focus);

[mwcampbell]

Please add a dummy change to platforms/winit/Cargo.toml (add or remove a blank line at the bottom), as we've done before, to force a semver-breaking bump to that crate, since we're doing such a bump to the accesskit crate. Then this will be ready to merge.