feat(ISSUE-4): add fullWidthSingleCell prop to structuredListCell

[Xin00163]

#690

What

1. Background - As a user, I would like to align the divider and content when there is only one cell in StructuredList so there is no extra padding/margin
2. What did you do - to avoid making a breaking change, I've added a prop to allow this to happen
3. What does the reviewers should expect - The rest of the scenarios should still work as expected. There is one extra scenario added to one cell.

I have done:

• Written unit tests against changes
• Written functional tests against the component and/or NewsKit site
• Updated relevant documentation

I have tested manually:

• The feature's functionality is working as expected on Chrome, Firefox, Safari and Edge
• The screen reader reads and flows through the elements as expected.
• There are no new errors in the browser console coming from this PR.
• When visual test is not added, it renders correctly on different browsers and mobile viewports (Safari, Firefox, small mobile viewport, tablet)
• The Playground feature is working as expected

Before:

After:

Who should review this PR:

How to test:

[pp-serviceaccount]

You can preview these changes on:
Documentation Site: http://ncu-newskit-docs-pr.s3-website-eu-west-1.amazonaws.com/issue-4-add-full-width-to-list-cell
Storybook: http://ncu-newskit-docs-pr.s3-website-eu-west-1.amazonaws.com/issue-4-add-full-width-to-list-cell/storybook

[pdimova]

👍