From https://make.wordpress.org/core/2021/06/29/block-based-widgets-editor-in-wordpress-5-8/#comment-41552:

No documentation how to add icons, no documentation how to add styles and scripts to legacy widget. No documentation how to make blocks only for Widgets, Customizer or for Post editor.

Most of the APIs for this are the same as the regular block editor's, but let's explicitly document it.