fix(heading-order): Crash on page with iframes but no headings

[WilcoFiers]

This PR is a rewrite of heading-order-after. It fixes a number of issues:

• When a window has frames, but no headings the headings should still be placed in the heading order. (This caused the errors of Runtime issue: Cannot read property 'data' of undefined #2928 and Runtime issue: Cannot read property 'data' of undefined #2944)
• The first result no longer automatically passes. It can fail if a heading before it was excluded, and there is a level-skip between them.
• Ancestor selectors in shadow DOM trees should be correctly recognised
• If for some reason the heading can't be found in the heading map, for example because of a custom selector, the rule no longer errors.

Closes issue: #2928 and #2944

[gdkraus]

@WilcoFiers, in reference to #2944 the description of what this change solves does not describe our situation. In our app, we have an iframe that has multiple headings in it.

[gdkraus]

@WilcoFiers, or do you mean if the outermost frame contains no headings, but contains iframes, that is when the problem occurs?

[gdkraus]

I manually edited our parent frame to have a single heading and ran axe DevTools again and it worked.