Main.js: Combine presets/registers in addons field

[shilman]

Issue: #9245

What I did

First implementation of a combined addons entry in main.js

Before: Presets are a “superset” of addons

• Addons are packages that extend Storybook’s UI (but not always, e.g. Storyshots). They sometimes use a mechanism called presets to configure storybook and/or register themselves (e.g. Docs)
• Presets are both packages that configure Storybook and also a mechanism by which addons can configure Storybook.

module.exports = {
  presets: ['@storybook/addon-docs/preset'],
  addons: ['@storybook/addon-actions/register']
}

After: Addons are a “superset” of presets

• Addons are packages that extend Storybook in various ways (UI, binaries, libraries, configurations)
• Presets are THE mechanism by which addons configure Storybook, including registering themselves with Storybook.

module.exports = {
  addons: [
    '@storybook/addon-docs',
    '@storybook/addon-actions'
  ]
}

Also the following works if you want to be more precise / backwards compatible:

module.exports = {
  addons: [
    '@storybook/addon-docs/preset',
    '@storybook/addon-actions/register'
  ]
}

Assumptions:

• Addon registration always ends in /register, e.g. @storybook/addon-actions/register. I've checked and this seems to be the case.
• If users want an addons function, they can use managerEntries instead.

How to test

See updated examples: official-storybook, vue-kitchen-sink, cra-ts-kitchen-sink

[vercel]

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[shilman]

self-merging @tmeasday @ndelangen