Make helper classes public

[sussexrick]

Would it be possible to make ExceptionHelper and TagHelperContextExtensions and DictionaryExtensions public? This would help when implementing your pattern to build more components.

Currently I've taken a copy of these to implement the proposed 'Task list' component.

Eventually I'd like to PR that component into this repo, but I imagine you'd want that to be after your v1.0 and after it's an approved component in the GOV.UK Design System. The PR for our repo is thepensionsregulator/govuk-frontend-aspnetcore-extensions#99 if you want to look and/or comment.