gpui: Allow selection of "Services" menu independent of menu title (#34115)

Release Notes:

- N/A

---

In the same vein as #29538, the "Services" menu on macOS depended on the
text being exactly "Services", not allowing for i18n of the menu name.

This PR introduces a new menu type called `OsMenu` that defines a
special menu that can be populated by the system. Currently, it takes
one enum value, `ServicesMenu` that tells the system to populate its
contents with the items it would usually populate the "Services" menu
with.

An example of this being used has been implemented in the `set_menus`
example:
`cargo run -p gpui --example set_menus`

---

Point to consider:

In `mac/platform.rs:414` the existing code for setting the "Services"
menu remains for backwards compatibility. Should this remain now that
this new method exists to set the menu, or should it be removed?

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/zed/src/zed/app_menus.rs

@@ -35,10 +35,7 @@ pub fn app_menus() -> Vec<Menu> {
                     ],
                 }),
                 MenuItem::separator(),
-                MenuItem::submenu(Menu {
-                    name: "Services".into(),
-                    items: vec![],
-                }),
+                MenuItem::os_submenu("Services", gpui::SystemMenuType::Services),
                 MenuItem::separator(),
                 MenuItem::action("Extensions", zed_actions::Extensions::default()),
                 MenuItem::action("Install CLI", install_cli::Install),