Heading: notify about wrong level more clearly

[mtias]

Right now we are exposing this functionality in the document outline tool:

This is good, but I think we can also explore exposing this in the block itself:

It shouldn't be overtly dramatic, but hinting at something being off:

[karmatosed]

I'm nodding as feel that is a great start. I would maybe consider the same treatment of a yellow background, or increasing the contrast on the coloring for headers (it feels like on the cusp of not being strong enough). I really like the idea of bringing awareness by the block itself.

[ZebulanStanphill]

I'm definitely in favor of doing this. One thing to consider is how we can make this notification accessible. Changing the toolbar button tooltip probably isn't the right approach. Would it be possible to show a similar warning as the color contrast warning in the heading level dropdown?

[Mamaduka]

Closing as duplicate of #10581. Both issues serve the same purpose.