chore: Remove outline dependency from breadcrumbs (#22504)

This slashes our incremental dev times (touch editor) by 0.6s
(8.1->7.6s) due to unblocking terminal_view build sooner.

Closes #ISSUE

Release Notes:

- N/A

crates/zed_actions/src/lib.rs

@@ -151,3 +151,13 @@ pub struct Rerun {
 }
 
 impl_actions!(task, [Spawn, Rerun]);
+
+pub mod outline {
+    use std::sync::OnceLock;
+
+    use gpui::{action_as, AnyView, WindowContext};
+
+    action_as!(outline, ToggleOutline as Toggle);
+    /// A pointer to outline::toggle function, exposed here to sewer the breadcrumbs <-> outline dependency.
+    pub static TOGGLE_OUTLINE: OnceLock<fn(AnyView, &mut WindowContext<'_>)> = OnceLock::new();
+}