Feature/1970 multi select props playground

[GCHQ-Developer-847]

Summary of the changes

Added multi-select props playground

I have also updated the validationStatus on the existing single / searchable select playgrounds to have an empty string option rather than setting it to a string with value "none"; this allows the validation status to be removed. (Also thought the select control would be more appropriate than radio buttons - for displaying an empty string).

Created #2044 for issue found during playground testing

Related issue

1970

Checklist

General

• [x] Changes to docs package checked and committed.
• [x] All acceptance criteria reviewed and met.

Testing

• [x] Relevant unit tests and visual regression tests added.
• [x] Visual testing against Figma component specification completed.

Accessibility

• [x] Accessibility Insights FastPass performed.
• [x] A11y unit test added and yields no issues.
• [x] A11y plug-in on Storybook yields no issues.
• [x] Manual screen reader testing performed using NVDA and VoiceOver.
• [x] Manual keyboard testing for keyboard controls and logical focus order.
• [x] Correct roles used and ARIA attributes used correctly where required.
• [x] Logical heading structure is maintained, and the HTML elements used for headings can be changed to fit within the wider page structure.

Resize/zoom behaviour

• [x] Page can be zoomed to 400% with no loss of content.
• [x] Screen magnifier used with no issues.
• [x] Text resized to 200% with no loss of content.
• [x] Text spacing increased as per the WCAG 1.4.12 success criterion with no loss of content.

System modes

• [x] Browser setting 'prefers reduced motion' tested. No animations or motion visible whilst this setting is on.
• [x] Windows High Contrast mode tested with no loss of content.
• [x] System light and dark mode tested with no loss of content.
• [x] Browser support tested (Chrome, Safari, Firefox and Edge).

Testing content extremes

• [x] Min/max content examples tested with no loss of content or overflow.
• [x] All prop combinations work without issue.
• [x] Tested for FOUC (Flash of Unstyled Content) in both SSR (Server-Side Rendering) and SSG (Static Site Generation) settings.
• [x] Controlled and uncontrolled input components tested.

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