Upgrade to Storybook 8

[allanlasser]

One of our plugins, storybook-addon-cookie, still needs to be updated for compatability with Storybook 8.

Version 8 removes peer-dependency on react and react-dom, so we won't need to include them in our package.json any more.

Version 8 also introduces changes and improvements to Storybook's testing tools.

I would recommend waiting to make this major version upgrade until after we've merged our sveltekit branch.