Heading block: add heading level checker#22650

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ZebulanStanphill wants to merge 3 commits intotrunkfrom add/heading-block-level-checker

+279-37

Commits on Mar 6, 2021

Original implementation by @Jackie6 (Rebased + major fixes/tweaks.)

ZebulanStanphill
and
Jackie6
committed

Commits on Mar 7, 2021