Decide which panel should be active for a dock based on ordering panels (#22346)

This means that `workspace::ToggleRightDock` will open the assistant if
no right-dock panel has been manually activated, instead of the chat as
before. Also cleans up the `active_panel_index` logic a bit.

cc @nathansobo 

Release Notes:

- Make `workspace::ToggleRightDock` open the assistant panel if no
right-dock panel has yet been activated

crates/zed/src/zed.rs

@@ -345,7 +345,7 @@ fn initialize_panels(prompt_builder: Arc<PromptBuilder>, cx: &mut ViewContext<Wo
             workspace.add_panel(channels_panel, cx);
             workspace.add_panel(chat_panel, cx);
             workspace.add_panel(notification_panel, cx);
-            workspace.add_panel(assistant_panel, cx);
+            workspace.add_panel(assistant_panel, cx)
         })?;
 
         let git_ui_enabled = git_ui_feature_flag.await;
@@ -380,7 +380,9 @@ fn initialize_panels(prompt_builder: Arc<PromptBuilder>, cx: &mut ViewContext<Wo
             } else {
                 workspace.register_action(assistant::AssistantPanel::inline_assist);
             }
-        })
+        })?;
+
+        anyhow::Ok(())
     })
     .detach();
 }