refactor: improve item tree display #1852

Job	Run time
Check	1m 20s
Test mdbook	1m 9s
Doctest Rust (ubuntu-latest)	1m 29s
Test C++ Runtime (ubuntu-latest)	3m 43s
Test Rust (ubuntu-latest)	4m 19s
Doctest Rust (windows-latest)	3m 51s
Check Style	1m 32s
Test Rust (windows-latest)	11m 4s
Test C++ Runtime (windows-latest)	6m 51s
Doctest Rust (macOS-latest)	3m 22s
Test C++ Runtime (macOS-latest)	5m 11s
Test Rust (macOS-latest)	5m 43s
	49m 34s

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