zed: Add ability to restore last session w/ multiple windows (#14965)

This adds the ability for Zed to restore multiple windows after
restarting. It's now the default behavior.

Release Notes:

- Added ability to restore all windows that were open when Zed was quit.
Previously only the last used workspace was restored. This is now the
default behavior. To get back the old behavior, add the following to
your settings: `{"restore_on_startup": "last_workspace"}` (Part of
[#4985](https://github.com/zed-industries/zed/issues/4985) and
[#4683](https://github.com/zed-industries/zed/issues/4683))

Demo:



https://github.com/user-attachments/assets/57a375ec-0c6a-4724-97c4-3fea8f18bc2d

---------

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

crates/workspace/src/workspace_settings.rs

@@ -47,8 +47,10 @@ pub enum RestoreOnStartupBehaviour {
     /// Always start with an empty editor
     None,
     /// Restore the workspace that was closed last.
-    #[default]
     LastWorkspace,
+    /// Restore all workspaces that were open when quitting Zed.
+    #[default]
+    LastSession,
 }
 
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
@@ -74,8 +76,8 @@ pub struct WorkspaceSettingsContent {
     /// Default: off
     pub autosave: Option<AutosaveSetting>,
     /// Controls previous session restoration in freshly launched Zed instance.
-    /// Values: none, last_workspace
-    /// Default: last_workspace
+    /// Values: none, last_workspace, last_session
+    /// Default: last_session
     pub restore_on_startup: Option<RestoreOnStartupBehaviour>,
     /// The size of the workspace split drop targets on the outer edges.
     /// Given as a fraction that will be multiplied by the smaller dimension of the workspace.