Add optional dart sass support

Problem/Motivation

Right now, when generating a theme with the generator script, it only generates css files.
Especially for color (scheme) definitions, font definitions, grid definitions etc. variables support would be great.

The alternatives like barrio sass are way too overheaded (For example: A generated own barrio sass sub theme depends on barrio sass theme which depends on barrio theme wich depends on bootstrap 5.x)

Proposed resolution

give the generator for the theme a (dart)sass (see https://sass-lang.com/dart-sass/) option with a predefined color scheme and a responsive grid. Perhaps something aside bootstrap to get here more diversity.
An option could be the soon arriving V1 of bulma framework (see https://github.com/jgthms/bulma/issues/3708)