Allow forms to have an exit page, e.g. for eligibility

### What feature do you want to see added?

Allow forms to have an exit page. This is where the user finishes the form, but does not submit it. This could be because they're not eligible for something.

See https://design-system.service.gov.uk/patterns/check-a-service-is-suitable/

### Why do you want this feature? Does it solve a problem?

This builds on the branching feature, and allows for more complex forms that assess if a user is eligible for a service.

This will save users from having to read through large amounts of documentation outside of the service to work out if they can use it.

### How should the feature be implemented?

See https://design-system.service.gov.uk/patterns/check-a-service-is-suitable/

This is a new option for the branching.

For simplicity, each prototype should only have one exit page with a singular message.

Consider if a page stating that a user is eligible should also be added.

### Additional context (if applicable)

This follows on from #41.

Start by reviewing the existing branching feature and issue #41, then compare the flow with the GOV.UK check-a-service-is-suitable pattern linked in the issue. Done means a prototype can branch to one exit page with a singular message without submitting the form; clarify whether an eligible-result page is also required.

Written by the indexing model from the issue text.