Implement "join pane into next" (#16077)

Closes #12409

Release Notes:

- Added "join pane into next" action ([#12409](https://github.com/zed-industries/zed/issues/12409))


https://github.com/zed-industries/zed/assets/727422/00cc8599-e5d6-4fb8-9f0d-9b232d2210ef

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/assistant/src/assistant_panel.rs

@@ -492,7 +492,7 @@ impl AssistantPanel {
         cx: &mut ViewContext<Self>,
     ) {
         let update_model_summary = match event {
-            pane::Event::Remove => {
+            pane::Event::Remove { .. } => {
                 cx.emit(PanelEvent::Close);
                 false
             }