message: chore: format output json with 2 spaces, like in input tree
  For easier comparison
diff: @@ -278,7 +278,7 @@ namespace Nextclade {
  TreeNode root() const;
  - std::string serialize(int spaces = 4) const;
  + std::string serialize(int spaces = 2) const;
  // Destructor is required when using pimpl idiom with unique_ptr.
  // See "Effective Modern C++" by Scott Meyers,
  