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/project_symbols/src/project_symbols.rs

@@ -127,13 +127,14 @@ impl PickerDelegate for ProjectSymbolsDelegate {
                     let position = buffer
                         .read(cx)
                         .clip_point_utf16(symbol.range.start, Bias::Left);
-
-                    let editor = if secondary {
-                        workspace.split_project_item::<Editor>(buffer, cx)
+                    let pane = if secondary {
+                        workspace.adjacent_pane(cx)
                     } else {
-                        workspace.open_project_item::<Editor>(buffer, cx)
+                        workspace.active_pane().clone()
                     };
 
+                    let editor = workspace.open_project_item::<Editor>(pane, buffer, cx);
+
                     editor.update(cx, |editor, cx| {
                         editor.change_selections(Some(Autoscroll::center()), cx, |s| {
                             s.select_ranges([position..position])