Start on adding support for editing via the assistant panel (#14795)

Note that this shouldn't have any visible user-facing behavior yet. The
feature is incomplete but we wanna merge early to avoid a long-running
branch.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/editor/src/editor_tests.rs

@@ -10445,7 +10445,12 @@ async fn test_mutlibuffer_in_navigation_history(cx: &mut gpui::TestAppContext) {
                 workspace.active_item(cx).is_none(),
                 "active item should be None before the first item is added"
             );
-            workspace.add_item_to_active_pane(Box::new(multi_buffer_editor.clone()), None, cx);
+            workspace.add_item_to_active_pane(
+                Box::new(multi_buffer_editor.clone()),
+                None,
+                true,
+                cx,
+            );
             let active_item = workspace
                 .active_item(cx)
                 .expect("should have an active item after adding the multi buffer");