Extend ContrastChecker with link-color support

[aristath]

Came up in #24513 (comment)

Currently ContrastChecker only checks for contrast between background-color and text-color.
With the introduction of link-color controls, we'll need to extend that functionality to link-colors as well.
Links however can be a little more complicated than normal text... Links should have sufficient contrast with the background color - just like normal text, but at the same time, they should have a minimum of 3:1 contrast with surrounding text.

See this article on WCAG and link-colors for more details on what the rules are for links.