Add grid overlay control to Storybook

[joshfarrant]

Summary

Adds a grid overlay control to the Storybook preview.

When building the RiverAccordion I needed an easy way to check that it conformed to our standard Grid, so I added this control.

The control supports multiple grid options to allow it to be used across all of our stories.

List of notable changes:

• Adds a grid overlay control to Storybook, which supports 5 modes
  • Off
  • Limited width — limits the grid to 1280px to match our standard Grid
  • Full width — allows the grid to expand to the full width of the viewport, similar to passing fullWidth to Grid
  • Limited width (no padding) — limits the grid to 1280px without padding
  • Full width (no padding) — allows the grid to expand to the full width of the viewport without padding
• Changes the icon for the colour mode control; a paint brush felt more appropriate

What should reviewers focus on?

• Are you happy with this addition?

Steps to test:

1. Open 🔗 Storybook
2. Click the Grid overlay control
   

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Example screenshots

[changeset-bot]

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Latest commit: c5f7fec

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[github-actions]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.