Add OpenExcerptsSplit (#8574)

I would like to keep diagnostics open on one side, and process them on
the other.


Release Notes:

- Added `editor::OpenExcerptsSplit` (bound to `cmd-k enter`) to open the
selected excerpts in the adjacent pane
- vim: Added `ctrl-w d`, `ctrl-w shift-d` and `ctrl-w space` for
`editor::GoTo{,Type}Definition` and `editor::OpenExcerptsSplit`

crates/activity_indicator/src/activity_indicator.rs

@@ -97,7 +97,7 @@ impl ActivityIndicator {
                             cx,
                         );
                     });
-                    workspace.add_item(
+                    workspace.add_item_to_active_pane(
                         Box::new(
                             cx.new_view(|cx| Editor::for_buffer(buffer, Some(project.clone()), cx)),
                         ),