Progress bar

This is a stub issue to add the progress bar that @edlerd @anasereijo

A pattern in Vanilla needs to be flexible enough that it can be used across a wide range of scenarios and shouldn't be constrained to a specific number of applications.

## Visual

Provide a screenshot or link to a prototype of your pattern proposal.

## Context

Where and when might this proposed pattern be used?

## State

Does this pattern have state? If so, what are the different states (e.g. accordion with closed/open states)?

## Progressive enhancement

A pattern should be designed small screen first. How does this pattern scale up?
How will this pattern degrade on less competent browsers?

Start by defining the progress bar proposal requested in the issue's Visual, Context, State, and Progressive enhancement sections. No implementation files or tests are named, so first identify where Vanilla patterns are documented and implemented. Done means an agreed visual proposal and a responsive, progressively enhanced progress-bar pattern.

Written by the indexing model from the issue text.