Port Firefox 121+'s Always Underline Links feature

[kfranqueiro]

Is your feature request related to a problem? Please describe. Many websites employ inaccessible designs which make links unnecessarily difficult or downright impossible to discern among other text.

Describe the solution you'd like Add an "Always Underline Links" option akin to what was added in Firefox 121.

Describe alternatives you've considered More-or-less doable by installing an extension capable of user styles e.g. Stylus, but AFAIK this can't override more specific (e.g. class) selectors with !important applied

Additional context Here are the relevant-looking changesets that added the feature and its UI to Firefox:

• https://phabricator.services.mozilla.com/D190671
• https://phabricator.services.mozilla.com/D190672