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)