Add "new window" option to the dock menu (#12067)

Fixes: #11651
Co-Authored-By: versecafe <147033096+versecafe@users.noreply.github.com>



Release Notes:

- Added a "New Window" item to the dock menu
([#11651](https://github.com/zed-industries/zed/issues/11651)).

---------

Co-authored-by: versecafe <147033096+versecafe@users.noreply.github.com>

crates/gpui/src/platform/test/platform.rs

@@ -233,6 +233,7 @@ impl Platform for TestPlatform {
     }
 
     fn set_menus(&self, _menus: Vec<crate::Menu>, _keymap: &Keymap) {}
+    fn set_dock_menu(&self, _menu: Vec<crate::MenuItem>, _keymap: &Keymap) {}
 
     fn add_recent_document(&self, _paths: &Path) {}