Add CSS variables for font weight

Discussed on https://github.com/xwiki/xwiki-platform/pull/3873 and https://forum.xwiki.org/t/css-best-practice-dont-hard-code-font-weights/14490

The LESS set of variables didn't include any standard value for font-weight. It only provided specific values for some elements (used in XWiki Bootswatch themes). However in a lot of cases we couldn't rely on those. That's why we want to introduce standard variables in the CSS system. This way, we won't have to hard code font-weights anymore (improves consistency and maintainability).