Draft: feat: add variables for jumbotron text transform

As discussed in hugo-solstice-theme!379, we need to allow consumers of EFSA/EFHST to control the casing of jumbotron headlines and subtitles without introducing breaking changes.

This patch adds new LESS variables to control the text-transform of these elements. It is intended only for Astro, where we can guarantee no website will be negatively affected by such a change.