Issue-3895: Make variables root configurable

[gabor-kormany]

This is a bugfix.

Issue-3895

Proposed solution

Make the css variables host configurable instead of hardcoded :root. This allows the usage where the css is expected to be scoped with a class and also in web components where the :root selector is not available, but :host is.

Tradeoffs

Testing Done

I've tested this be creating a test.scss file with the following content:

@use "sass" with (
  $variables-host: ':host'
);

Made the output css file variables under :host as expected.

Changelog updated?

No.