Add CSS to progress bar [#3514810]

Problem/Motivation

The progress bar has a lot of nice functionality but lacks any CSS so i'm wondering if we can add some css to the progress bar.

Steps to reproduce

Proposed resolution

I was thinking of using the Progress Tracker library similar to how it is done in the webform module.
Classes for the progress bar will need to be added to the twig, and the css will need to be added to the libraries.

Remaining tasks

User interface changes

API changes

Data model changes

Issue fork forms_steps-3514810

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3514810-add-css-to changes, plain diff MR !21
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