Implement 'Cmd+W with no open tabs closes the window', with a setting (#11989)

Follow up to: https://github.com/zed-industries/zed/pull/10986

However, I have set this to have a default behavior of 'auto': matching
the current platform's conventions, rather than a default value of
'off'.

fixes https://github.com/zed-industries/zed/issues/5322.

Release Notes:

- Changed the behavior of `workspace::CloseActiveItem`: when you're
using macOS and there are no open tabs, it now closes the window
([#5322](https://github.com/zed-industries/zed/issues/5322)). This can
be controlled with a new setting, `when_closing_with_no_tabs`, to
disable it on macOS, or enable it on other platforms.

crates/workspace/src/pane.rs

@@ -5,7 +5,7 @@ use crate::{
     },
     toolbar::Toolbar,
     workspace_settings::{AutosaveSetting, TabBarSettings, WorkspaceSettings},
-    NewCenterTerminal, NewFile, NewSearch, OpenInTerminal, OpenTerminal, OpenVisible,
+    CloseWindow, NewCenterTerminal, NewFile, NewSearch, OpenInTerminal, OpenTerminal, OpenVisible,
     SplitDirection, ToggleZoom, Workspace,
 };
 use anyhow::Result;
@@ -948,6 +948,14 @@ impl Pane {
         cx: &mut ViewContext<Self>,
     ) -> Option<Task<Result<()>>> {
         if self.items.is_empty() {
+            // Close the window when there's no active items to close, if configured
+            if WorkspaceSettings::get_global(cx)
+                .when_closing_with_no_tabs
+                .should_close()
+            {
+                cx.dispatch_action(Box::new(CloseWindow));
+            }
+
             return None;
         }
         let active_item_id = self.items[self.active_item_index].item_id();