feat(headings): add warnings for hidden heading levels

[ericrallen]

I wasn't entirely sure what the resolution for #121 should be, or why there would be hidden heading levels on the page, but this update treats a heading with aria-hidden or that meets the criteria for the :hidden selector as a Warning and styles things differently for them.

Submitted as a Draft PR because I'm not even sure if this is something that should be added or if you'd prefer a more robust solution that adds a Warnings tab to the Info Panel.

Closes #121

[khanbot]

CLA signature looks good 👍

[somewhatabstract]

I haven't forgotten this. I will try to make time this week to give feedback. Thank you for your patience.

[ericrallen]

Sure thing. I just picked this one up because I had some free time and saw an issue I thought I could handle.

Like I said, it might not even be worth pursuing.

[somewhatabstract]

This is a nice clean change.

It needs to be merged up with the latest version of master of course, but otherwise looks great.

[somewhatabstract]

Having the idea of warnings for this makes sense. I mean, hidden headers could be a mistake or it could be intentional.

I wonder if we should add a feature to control what level of issue we annotate? Like a cumulative level thing that says "show all annotations of level X and above"? What do you think? (not suggesting you do that as part of this PR, just thinking aloud).

[ericrallen]

I think that definitely makes sense if you introduce levels.

It could work sort of like the Console log-level filtering in your browser's Developer Tools.

It might even make sense to have each plugin be able to define what its defaults are and fall back to some global default of ["error", "warning"] if no levels are specified by the plugin.