Add Trigger

[kelliedesigner]

As a user of the design system I want to know how to provide a Trigger So that there is a consistent experience across services

Links and resources

Acceptance criteria

• [x] Add component or pattern
• [x] Cross-browser test
• [x] Accessibility test
• [x] Add changelog entry

[kelliedesigner]

https://design-system.barnardos.org.uk/components/forms/#trigger