Make Interruption panel more consistent with other components

[NickColley]

What

Consider more B.E.M. aligned markup for Interruption panel

Why

Cookie banner and Summary list has a similar concept of actions and we do that using Nunjucks rather than using deep CSS selectors, should we

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• Decide if we want a Nunjucks interface like Cookie banner
• Decide if we want to inject a modifier class for Button group or have it outside the body in the HTML
• Nunjucks Parameters have a content review

[NickColley]

Initial spike for review here, pretty scrappy but I think enough to chat:
alphagov/govuk-frontend#7246

[NickColley]

We decided to go for it and here's the proper go at it:
alphagov/govuk-frontend#7262 (review)