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/command_palette/src/command_palette.rs

@@ -468,7 +468,7 @@ mod tests {
         });
 
         workspace.update(cx, |workspace, cx| {
-            workspace.add_item(Box::new(editor.clone()), cx);
+            workspace.add_item_to_active_pane(Box::new(editor.clone()), cx);
             editor.update(cx, |editor, cx| editor.focus(cx))
         });