Widget Importer: ensure it works with the widget group block

[annezazu]

What problem does this address?

In writing the next call for testing for the FSE Outreach Program, I created some widgets in a classic theme before switching my site to a block theme and noticed that the import feature doesn't work with the Widget Group block:

What is your proposed solution?

Ensure compatibility with the widget group block, especially since this block was made to ensure an easier experience working with the block based widget screen: https://make.wordpress.org/core/2021/09/22/core-editor-improvement-introducing-the-widget-group-block/ cc @Mamaduka

[annezazu]

Listing this as a bug instead of an enhancement as this feels more like broken behavior. If that feels off, let me know!