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

Cargo.lock

@@ -8515,6 +8515,7 @@ dependencies = [
  "serde_derive",
  "serde_json",
  "settings",
+ "smallvec",
  "theme",
  "ui",
  "util",