add javascript contrast watcher

[Bubballoo3]

In draft while I work out the kinks, but this seems to be the best approach for checking contrast without adding dependencies.

The basic premise is that we attach this snippet of javascript to every 'visit' call, ensuring that it runs on all tests without needing modification. From there, the javascript

• reads every element, checking if the element is valid, visible, and contains text directly (not a child with text)
• reads the computed styles, deferring to parents when it encounters a transparent background
• raises a javascript error when a violating element is found, which fails the test and shows the javascript error (with info about the offending element).

I have only tested this in limited contexts and will no doubt encounter edge cases when it is run with all the tests. However I think the potential benefits are real and that in the long run this will be worth developing and maintaining.