New css variable option for link underlines

Created by: AgentSmith0

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

What about a variable for enabling/disabling the link underline? https://twbs-bootstrap.netlify.app/docs/5.1/customize/options/

Motivation and context

Many pages disable the underline on links and have link highlighting only by color. With this option this could be easily customized.