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/ui/src/components.rs

@@ -8,6 +8,7 @@ mod dropdown_menu;
 mod facepile;
 mod icon;
 mod image;
+mod indent_guides;
 mod indicator;
 mod keybinding;
 mod label;
@@ -40,6 +41,7 @@ pub use dropdown_menu::*;
 pub use facepile::*;
 pub use icon::*;
 pub use image::*;
+pub use indent_guides::*;
 pub use indicator::*;
 pub use keybinding::*;
 pub use label::*;