call rebuild on the form_state with every preview click

Really nice module this, makes editors' jobs easier knowing what a paragraph will look like before saving content, which i think is important.

One thing I found was that if i edited a value on a paragraph (once i already created it), the preview wouldn't show the updated field values unless you select one of the paragraph actions like `add another paragraph` or `collapse` etc.

I found that in `submitPreviewerItem` in `ParagraphsPreviewerWidgetTrait` it was checking that if the form was not cached and then it would call $form_state->rebuild() .

Can we remove this check and just call setRebuild each time someone selects the preview link? I don't think there will be much of a performance loss from doing this and it means that when values are altered, the preview option will be able to reflect updated values straight away.

I will attach a patch.

Let me know your thoughts