Update Storybook to latest (6.0+)

[haszari]

Describe the bug
There's a major new version of Storybook, with some cool new features. Since the API is slowly evolving, I think it's a good idea to keep the dependency up to date.

We might need more work to adopt all the new features/APIs - this issue is just to get on the latest version.

https://medium.com/storybookjs/storybook-6-migration-guide-200346241bb5

https://github.com/storybookjs/storybook/blob/next/MIGRATION.md

Potential follow up issues

These are not in order - there are possible dependencies between items, this might need a coordinated plan of effort.

• Adopt Storybook Essentials - well supported bundle of recommended addons.
• Build and publish storybook on every merge to master. This could potentially replace or supplement the existing component docs, but we'd need more Storybook coverage to replace (see next item!).
• Improve Storybook coverage - add stories for all components.
• Adopt MDX format for stories, so docs make intention and intended usage/limitations clear.
• Adopt args-based controls and component interface docs, so stories are more complete and easier (less manual) to keep up to date.

Related issues in other repos

• Improve storybook coverage for general-purpose components woocommerce-blocks#3031
• Storybook: Next steps for better experience when developing UI components WordPress/gutenberg#17973

[haszari]

@timmyc is there anyone who I could pass the Isotope Storybook torch to? Would be good to chip away at coverage and/or get it building off master branch so it becomes vibrant and alive over time. I haven't logged issues for any of the possible follow up above - let me know if I can help in any way :)