Support macOS Sequoia titlebar double-click action (#30468)

Closes #16527

Release Notes:

- Added MacOS titlebar double-click action

---

Unfortunately, Apple doesn't seem to make the "Fill" API public or
documented anywhere.

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/gpui/src/platform.rs

@@ -445,6 +445,7 @@ pub(crate) trait PlatformWindow: HasWindowHandle + HasDisplayHandle {
     // macOS specific methods
     fn set_edited(&mut self, _edited: bool) {}
     fn show_character_palette(&self) {}
+    fn titlebar_double_click(&self) {}
 
     #[cfg(target_os = "windows")]
     fn get_raw_handle(&self) -> windows::HWND;