project panel: Add indent guides (#18260)

See #12673



https://github.com/user-attachments/assets/94079afc-a851-4206-9c9b-4fad3542334e



TODO:
- [x] Make active indent guides work for autofolded directories
- [x] Figure out which theme colors to use
- [x] Fix horizontal scrolling
- [x] Make indent guides easier to click
- [x] Fix selected background flashing when hovering over entry/indent
guide
- [x] Docs

Release Notes:

- Added indent guides to the project panel

crates/project_panel/src/project_panel_settings.rs

@@ -20,6 +20,7 @@ pub struct ProjectPanelSettings {
     pub folder_icons: bool,
     pub git_status: bool,
     pub indent_size: f32,
+    pub indent_guides: bool,
     pub auto_reveal_entries: bool,
     pub auto_fold_dirs: bool,
     pub scrollbar: ScrollbarSettings,
@@ -71,6 +72,10 @@ pub struct ProjectPanelSettingsContent {
     ///
     /// Default: 20
     pub indent_size: Option<f32>,
+    /// Whether to show indent guides in the project panel.
+    ///
+    /// Default: true
+    pub indent_guides: Option<bool>,
     /// Whether to reveal it in the project panel automatically,
     /// when a corresponding project entry becomes active.
     /// Gitignored entries are never auto revealed.