project panel: Add setting to disable auto opening project panel (#34752)

Release Notes:

- Add `project_panel.starts_open` to control opening project panel in
new projects.

crates/project_panel/src/project_panel_settings.rs

@@ -43,6 +43,7 @@ pub struct ProjectPanelSettings {
     pub sticky_scroll: bool,
     pub auto_reveal_entries: bool,
     pub auto_fold_dirs: bool,
+    pub starts_open: bool,
     pub scrollbar: ScrollbarSettings,
     pub show_diagnostics: ShowDiagnostics,
     pub hide_root: bool,
@@ -139,6 +140,10 @@ pub struct ProjectPanelSettingsContent {
     ///
     /// Default: true
     pub auto_fold_dirs: Option<bool>,
+    /// Whether the project panel should open on startup.
+    ///
+    /// Default: true
+    pub starts_open: Option<bool>,
     /// Scrollbar-related settings
     pub scrollbar: Option<ScrollbarSettingsContent>,
     /// Which files containing diagnostic errors/warnings to mark in the project panel.