Allow editing of confirmation page

[jenbutongit]

Is your feature request related to a problem? Please describe. The designer does not have an interface for editing the confirmation page, however it is editable/customisable via JSON (#573)

Describe the solution you'd like Add an edit confirmation page to the designer.

This is the type definition of a confirmation page.

type Toggleable<T> = boolean | T;

 type ConfirmationPage = {
   customText: {
     title: string;
     paymentSkipped: Toggleable<string>;
     nextSteps: Toggleable<string>;
   };
   components: ComponentDef[];
 };

 export type SpecialPages = {
   confirmationPage?: ConfirmationPage;
 };

Each of these properties must be editable

Describe alternatives you've considered

Additional context

[adamdavies1]

https://github.com/XGovFormBuilder/digital-form-builder/pull/700 This has been partly competed, designer still needs an edit part.