Add preview mode for checkboxes

[TomasEng]

Description

Added an "mvp" version of preview mode for checkboxes. I also added a button in the form builder to switch to preview mode for this component. We can change that later when we have implemented preview for all components and it becomes the default state, but in the meantime I think this is a good solution while we are transitioning to the new design. It allows users to test out the new view while the old one is still available.

This pull request does not include editing texts directly in the preview. I think we need to discuss with the designers exactly how this functionality should appear. It may be confusing for the users to see the same text editor appear twice (both in the middle section and in the right section).

Related Issue(s)

• Form builder should look closer to production #9218

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
  It works, but there are some issues in the design system's CheckboxGroup component. Labels should not listen to click events in presentation mode (since there are buttons inside the labels), and there is also some weird behaviour when selecting a checkbox.
• Relevant automated test added (if you find this hard, leave it and we'll help out)
• All tests run green

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)