Show duplicative heading level text when showIconLabels is enabled

[richtabor]

As a follow-up to #56035, instead of adding the duplicative visible text of Heading $level next to the corresponding H $level icon, let's enable the visible text when showIconLabels is enabled.

[carolinan]

I disagree and I don't consider this a "follow-up", but a suggestion to revert the result of the discussion.
The benefit of visible text has been clarified in #56035