Rework context insertion UX (#12360)

- Confirming a completion now runs the command immediately
- Hitting `enter` on a line with a command now runs it
- The output of commands gets folded away and replaced with a custom
placeholder
- Eliminated ambient context

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b1927a45-52d6-4634-acc9-2ee539c1d89a">

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/gpui/src/elements/div.rs

@@ -291,6 +291,8 @@ impl Interactivity {
                 let action = action.downcast_ref().unwrap();
                 if phase == DispatchPhase::Capture {
                     (listener)(action, cx)
+                } else {
+                    cx.propagate();
                 }
             }),
         ));