Check non-text contrast

### Product

axe-core

### Feature Description

This is part of WCAG21 https://www.w3.org/TR/WCAG21/#non-text-contrast.

It looks like `color-contrast` and `color-contrast-enhanced` only check elements with text in them based on their documentation. If they do also test graphical elements then the docs need updating.

I imagine this hasn't been implemented because it's hard to tell if an empty element is intended to be graphical. It would at least be nice to have the option to run a checker on elements we know are graphical, for example we have a "status dot" that can be different colours and appears on different colour backgrounds, it would be great if we could check its contrast everywhere it appears.