title_bar: Remove dependency on extensions_ui (#20929)

This PR removes a dependency on the `extensions_ui` from the `title_bar`
crate.

This dependency only existed to reference the `Extensions` action, which
has now been moved to the `zed_actions` crate.

This allows `title_bar` to move up in the crate dependency graph.

Release Notes:

- N/A

crates/zed/src/zed/app_menus.rs

@@ -32,7 +32,7 @@ pub fn app_menus() -> Vec<Menu> {
                     items: vec![],
                 }),
                 MenuItem::separator(),
-                MenuItem::action("Extensions", extensions_ui::Extensions),
+                MenuItem::action("Extensions", zed_actions::Extensions),
                 MenuItem::action("Install CLI", install_cli::Install),
                 MenuItem::separator(),
                 MenuItem::action("Hide Zed", super::Hide),