feat(helper/toc): specify maximum number of items to output (#5487)

* feat(helper/toc): specify maximum number of items to output

* perf: skip evaluation if `max_items` is not specified

* perf: streamline getting min and max heading level

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Co-authored-by: Uiolee <22849383+uiolee@users.noreply.github.com>
Co-authored-by: yoshinorin <yoshinorin.net@outlook.com>