Switch from Gulp build process to something else

Gulp has some registry system for better use/sharing of tasks, worth of checking https://gulpjs.com/docs/en/advanced/creating-custom-registries/.

Drupal 10 switched to postcss in core themes with some simple js scripts that are executed with node from package.json with.

Still I don't like fully just postcss approach, it is great for autoprefixer support and other similar stuff, but sass still looks much better.

For now I think the best options are:

• try not to complicate stack more, so avoid webpack and other things for now
• use sass dart implementation
• start adding css variables which are supported by sass variables
• use postcss the same as we are using it now
• replace gulp with custom js code for compiling and building css - basically we need to replace current build-css task "npx gulp clean:css && npx gulp sass" with something else
• use package.json scripts (as we are doing now) to execute various tasks