Support storybook 4.x

[John Schulz]

Storybook has released a new major version

I didn't see a migration guide for addon authors, but maybe there's some useful in these

• https://medium.com/storybookjs/migrating-to-storybook-4-c65b19a03d2c
• https://github.com/storybooks/storybook/blob/master/MIGRATION.md
• https://storybook.js.org/addons/writing-addons/#the-addon