Description of the Change

This PR is a follow-up to PR #126, as discussed here.

It removes group visibility settings from the block sidebar and adds groups as an inner block to allow reordering.

Additionally, the PR changes the default value of some attributes from static values to values from the global settings.

How to test the Change

1. Insert the "Mailchimp List Subscribe Form" block in the editor and verify that the group's visibility is pre-selected based on the current settings.
2. Reorder the groups, save the block, and check that the front end displays the groups in the configured order.
3. Update the groups' visibility and ensure the changes are accurately reflected on the front end.
4. Add or remove groups in the Mailchimp settings, then go to wp-admin Mailchimp settings, click "Update List" to fetch the latest groups, edit the block, and verify that the groups update accordingly.

Changelog Entry

N/A

Credits

Props @username, @username2, ...

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.