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/theme/src/styles/colors.rs

@@ -123,6 +123,9 @@ pub struct ThemeColors {
     pub search_match_background: Hsla,
     pub panel_background: Hsla,
     pub panel_focused_border: Hsla,
+    pub panel_indent_guide: Hsla,
+    pub panel_indent_guide_hover: Hsla,
+    pub panel_indent_guide_active: Hsla,
     pub pane_focused_border: Hsla,
     pub pane_group_border: Hsla,
     /// The color of the scrollbar thumb.