Move bundling of Worker under a flag

[gbmatt]

Add support for turning off bundling of Worker, for situations where we only need to bundle SharedWorker.

Basically, it allows projects that want to use SharedWorker (and don't need Worker) to use this plugin without facing issues such as https://github.com/GoogleChromeLabs/worker-plugin/issues/12

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[developit]

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[gbmatt]

@googlebot I signed it!

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[developit]

Published in 4.0.1!