link-in-text-block should consider surrounding node for color contrast

At the moment, Link-in-text-block only looks at the parent block element's background and foreground color when it tries to determine the 3:1 contrast. It should also look at the surrounding text to determine that contrast. For example:

```html

While on the road to my home of sweet Athy

• Welcome to the jungle


• We've got fun


• and games

```