fix: support rollup plugins in manager.js

[castastrophe]

What I did

I noticed that running build-storybook was not reading in the rollup image plugin necessary for theme.js to load a brand image. No issues with this in the web-dev-server command for running the local preview. This adds processing for the rollup plugins for the manager assets as well.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: e186df0a7b034532b1d14a4021b15bba02148db0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@web/dev-server-storybook	Patch

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[LarsDenBakker]

I think it's better to introduce a new option for this, as most plugins should be used for the preview (that's where the user's own code is). Plugins for the preview might conflict with the manager code, or at the least make it slower.