Addon-backgrounds: Simplified parameters API

[shilman]

See discussion here #9164

// preview.js
addParameters({
  backgrounds: {
    default: 'white',
    values: [
      { name: 'white', value: '#ffffff' }, 
      { name: 'black', value: '#000000' },
    ]
  }
}

// Foo.stories.js
export const Foo = ...
Foo.story = {
  parameters: {
    backgrounds: { default: 'black' },
  }
}

Needed:

• Upgraded API
• Deprecate old API (or remove completely)
• Update documentation
• Update MIGRATION.md

[shilman]

Yippee!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.0.0-beta.3 containing PR #10634 that references this issue. Upgrade today to try it out!

You can find this prerelease on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.