API: Add method to set config for manager

[ndelangen]

Issue:
Setting theme via parameters is really bad for performance
This is true for other config too.

What I did

ADD a setConfig option on lib/addons, to pass data/config into the manager directly VS via parameters

This PR adds a API:

import { addons } from '@storybook/addons';
import { themes } from '@storybook/theming';

addons.setConfig({
  theme: themes.dark,
});

You set this within addons.js.

How to test

I've changed the dev-kits example to dark theme.

There's no flash of light theme anymore.

[ndelangen]

I implemented the setting of theme, but all other configs will still have to be implemented.

Looking forward to hearing your feedback @shilman

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[shilman]

@ndelangen Super interesting, and raises lots of questions:

• Are these settings also available in the preview? (And vice versa)
• Which settings should be set this way vs. the old way?
• Do we want to rename addons.js to manager.js?
• What's the migration path for users?
• Is this a breaking change?

[ndelangen]

Are these settings also available in the preview? (And vice versa)

No, parameters are shared meta-data on stories, those are shared

Which settings should be set this way vs. the old way?

If the data is for the manager only, parameters only make sense if the data is related/attached to a story

Do we want to rename addons.js to manager.js?

Would probably be a good change to make, that would be a breaking change though

What's the migration path for users?

We'd need to change the documentation & our examples.
I don't think a codemod is viable. So much stuff can go down in config.js...
We can however add a deprecation and a warning how to change config.js to remove the custom theme, and how to add it to addons.js.

Is this a breaking change?

No, not as it is right now.

[shilman]

I understand it & it's a pretty cool change! Not exactly sure what's the best way to communicate it to users, but it's a big perf win.

[ndelangen]

Addresses:
#8188

[ndelangen]

It would be nice to get some stricter types on this.