Color contrast should check first-letter and first-line pseudo-elements

Product: axe-core

Expectation: first-letter and first-line pseudo-elements should be checked for color-contrast failures

Actual: Color applied via first-letter or first-line CSS is ignored

Motivation: first-letter is used in trade book publishing to replicate drop caps, first-line is less prevalent. We've implemented axe-core as part of our ebook production process, and these color contrast issues have to be checked manually (or are being missed). It is also an issue in [ace by daisy](https://github.com/daisy/ace) (we use both).

Github doesn't accept html files as attachments, attached examples as txt instead
[first-letter pseudo-element.txt](https://github.com/dequelabs/axe-core/files/6232477/first-letter.pseudo-element.txt)
[first-line pseudo-element.txt](https://github.com/dequelabs/axe-core/files/6232478/first-line.pseudo-element.txt)