4.0 dev relative sizes

[hans2103]

Pull Request for Issue # .

Summary of Changes

This PR will make the Cassiopeia more a11y by replacing sizes set in px by relative sizes like em and rem.
An interesting blog explaining why we have to do this can be written on 24a11y.com => https://www.24a11y.com/2019/pixels-vs-relative-units-in-css-why-its-still-a-big-deal/

Testing Instructions

• open Joomla 4 test and inspect elements... font-size, breakpoints and site--grid are set in PX
• open browser settings and increase default font-size.
• see result
• open browser settings and set default font-size back to normal
• apply PR and run npm run build:css
• open Joomla 4 test and inspect elements... font-size, breakpoints and site--grid are set in EM
• open browser settings and increase default font-size.
• see result

If all sizes are set in relative sizes the website grows equally when default font-size is enlarged or reduced

[richard67]

The changes in this PR have been integrated into #41 .