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/workspace/src/persistence/model.rs

@@ -360,7 +360,9 @@ impl SerializedPaneGroup {
                 } else {
                     let pane = pane.upgrade()?;
                     workspace
-                        .update(cx, |workspace, cx| workspace.force_remove_pane(&pane, cx))
+                        .update(cx, |workspace, cx| {
+                            workspace.force_remove_pane(&pane, &None, cx)
+                        })
                         .log_err()?;
                     None
                 }