outline panel: Add indent guides (#19719)

See #12673

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Release Notes:

- Added indent guides to the outline panel

Cargo.lock

@@ -7728,8 +7728,10 @@ dependencies = [
  "serde",
  "serde_json",
  "settings",
+ "smallvec",
  "smol",
  "theme",
+ "ui",
  "util",
  "workspace",
  "worktree",