Investigate removing CJS outputs for client-only entry points

[JReinhold]

When going through the exports in @storybook/core's package.json, the following entry points stands out, because they should theoretically only ever be used in the browser, so there is no need to bundle and distribute a CJS version of them as well:

• /client-logger
• /preview-errors
• /preview-api
• /manager-api
• /router
• /components
• /theming
• /theming/create

Collectively these CJS files make up 2.2 MB of the final bundle.

Notice that the manager-errors entry point already does not contain a CJS condition.

I might be missing some crucial context about how Webpack bundles, or how this all work together with Portable Stories (especially in Jest)

[JReinhold]

Closing as these entry points might be used in portable stories (in Jest, CJS), which is why they're needed. It would be strange to use these dependencies in stories, but it is possible when eg. users are using DocsContext from addon-docs in a decorator or similarly.