[BCL builder] Make postcss prefixer option more flexible

[vever001]

Problem

BCL builder allows passing the prefix option when compiling sass files, but it is not flexible enough as it only passes the prefix and not the entire postcss-prefix-selector config which we would need.

Also, the plugin gets run with an empty prefix when no prefix are specified, which I believe is slowing down the compilation for no reason. And the current transform callback is not necessary and optional.

Context:

We would need such feature/flexibility for ckeditor5 stylesheets which expects all CSS to be prefixed with ".ck-content", but we need to specify the transform callback to deal with ":root". Also relates to https://github.com/openeuropa/oe_bootstrap_theme/issues/404 To be:

{
  entry: path.resolve(baseDir, "resources/sass/style.scss"),
  dest: path.resolve(baseDir, "assets/css/ckeditor5-style.css"),
  options: {
    ...
    prefixer: {
      prefix: ".ck-content",
      // Replace ":root" with prefix.
      transform: (prefix, selector, prefixedSelector) => {
        if (selector.includes(":root")) {
          return selector.replace(/:root/g, prefix);
        }
        return prefixedSelector;
      }
    }
  }
}

Proposed resolution

Deprecate the prefix option in favor of prefixer which holds the entire prefixer config. Run the plugin only when one of these options is passed.