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/title_bar/src/application_menu.rs

@@ -121,8 +121,16 @@ impl ApplicationMenu {
                                     menu.action(name, action)
                                 }
                                 OwnedMenuItem::Submenu(_) => menu,
+                                OwnedMenuItem::SystemMenu(_) => {
+                                    // A system menu doesn't make sense in this context, so ignore it
+                                    menu
+                                }
                             })
                     }
+                    OwnedMenuItem::SystemMenu(_) => {
+                        // A system menu doesn't make sense in this context, so ignore it
+                        menu
+                    }
                 })
         })
     }