workspace: Add restore_on_startup setting that allows always opening an empty Zed instance (#10366)

Fixes #7694
The new setting accepts "last_workspace" (default) and "none" as
options.

In a follow-up PR I'll add a new option that re-launches all of the Zed
windows and not just the last one.

Release Notes:

- Added `restore_on_startup` option, accepting `last_workspace`
(default) and `none` options. With `none`, new Zed instances will not
restore workspaces that were open last.

crates/workspace/src/workspace.rs

@@ -81,7 +81,9 @@ use ui::{
 };
 use util::ResultExt;
 use uuid::Uuid;
-pub use workspace_settings::{AutosaveSetting, TabBarSettings, WorkspaceSettings};
+pub use workspace_settings::{
+    AutosaveSetting, RestoreOnStartupBehaviour, TabBarSettings, WorkspaceSettings,
+};
 
 use crate::notifications::NotificationId;
 use crate::persistence::{