feat/6157: step by step pattern

restored the px-to-em sass helper as it is used by the step by step navigation pattern
added the step by step navigation sass and js files from the GOV UK repo
changed the all.mjs and all.scss files to include the step by step navigation styling and scripts
refactored the commonPageComponenets.createBody() helper to have additional customisation
refactored current usage of createBody() in existing stories to match new function signature
refactored the preview-body.html so that stories work in both the Canvas tab and full page mode of storybook
created story for the step by step navigation component
created an example page using the step by step navigation component

ogcio / ogcio-ds