Addon-themes: Migrate to story globals

[shilman]

Describe the problem

Describe the problem

Story globals simplifies how to configure globals at the story level. We should update the Themes addon to use the new scheme.

• Figure out a final API
  • globals for storing/accessing state
• Update documentation

Before

export const Story = {
  parameters: {
    themes: {
      default: 'twitter',
      list: [
        { name: 'twitter', class: ['theme-twt', 'light-mode'], color: '#00aced' },
        { name: 'facebook', class: ['theme-fb', 'dark-mode'], color: '#3b5998' },
      ],
    },
  },
};

After (TBD)

export const Story = {
  globals: {
    theme: 'twitter',
  },
};

Additional context

We should also try to reconcile Chromatic Modes if we can

Additional context

No response