Legacy widget preview may not work as expected because it lacks front end assets

[jorgefilipecosta]

Describe the bug
Some widgets have a backend UI on the widget screen but don't provide the scripts/styles that render that content on the frontend.
In the editor, we don't load all the frontend assents so the preview may not work as expected.

A possible solution is to render the widget previews on an iframe with all the frontend assets.
PR #14643 implemented this solution but did not get much traction.

To reproduce
I installed https://wordpress.org/plugins/instagram-slider-widget/
I tested the widget with an Instagram hashtag and verified the widget preview does not contain the slider UI that appears on the frontend.

[noisysocks]

May be fixed by #14643.