Evaluate migrating from SASS to PostCSS or Lightning CSS

[mfranzke]

We should evaluate to switch from SCSS to Lightning CSS or expanding our usage of PostCSS, enjoying the following benefits:

• We could write modern CSS, while having it transferred to a compatible version.
• Besides that we could provide the our CSS source files, as these might be possible to get consumed out of the box by applications, that only need to support chromium based browsers in their context.
• We wouldn't need any style related build-step on our localhosts, a soft-reload could happen instantly.

The following CSS functionality, that is quite nicely supported by browsers even already, would benefit from not being transformed by SCSS anymore:

• CSS nested selectors

Besides that, we could use the following CSS standard functionality:

• CSS if() function
• CSS @function

We need to evaluate whether both PostCSS and Lightning CSS provide a way to aggregate our separated, modular CSS files into one with a similar concept of SCSS @use (or @import).