Description of the Change

The PR updates the Mailchimp form block to include settings for selecting an audience list, reordering fields, toggling field and group visibility, adjusting padding and margin, typography settings, and modifying colors. The following major changes were made as part of these block updates:

Block Updates:

• Updated the header and subheader markup to H2 and H3, and made them editable within the block.
• Added list selection to the block settings and added toggle options for group visibility in the block sidebar.
• Added form field reordering and field visibility controls. Made the form field labels editable.
• Added toggles for "Double Opt-In," "Update Existing Subscribers," and "Include Unsubscribe Link" under the form settings in the block sidebar.
• Made the "Subscribe" button text and unsubscribe link text editable.
• Added various settings such as color, typography, dimensions, and border under the styles tab in the block sidebar.

Settings Page Updates:

• Removed the "CSS Cheat Sheet" section.
• Removed the "Use JavaScript Support?" option.
• Removed the "Use JavaScript Datepicker?" option.
• Removed the "Custom Styling" settings (these settings will continue to work for users who have enabled them).
• Removed the "Remove Mailchimp CSS" settings (these settings will continue to work for users who have enabled them).

Closes #82

How to test the Change

1. Insert the "Mailchimp List Subscribe Form" block in the editor and verify that it automatically selects the list configured in general settings, with merge fields visibility pre-selected based on the current settings.
2. Update the header and subheader, then confirm that the changes appear correctly on the front end.
3. Reorder the form fields, save the block, and check that the front end displays the fields in the configured order.
4. Update the form fields' visibility and ensure the changes are accurately reflected on the front end.
5. Add or remove merge fields in the Mailchimp settings, then go to wp-admin Mailchimp settings, click "Update List" to fetch the latest form fields, edit the block, and verify that the form fields update accordingly.
6. Toggle the visibility of the groups from the block sidebar and verify that the changes are visible both in the editor and on the front end.
7. Experiment with various style options from the Styles tab to ensure they function as expected.
8. Change the list from the block sidebar and verify that the subscribe form updates with the merge fields corresponding to the selected list.
9. Test the subscribe process using different scenarios to confirm that the form works properly, provides the expected validation or error messages, and subscribes users to the correct list.
10. Update the toggles for "Double Opt-In," "Update Existing Subscribers," and "Include Unsubscribe Link" in the form settings, and verify that they operate correctly.

Backward Compatibility:

1. Ensure that the existing shortcode and widget continue to function without issues.
2. Edit an existing block and verify that it updates correctly with the form fields and settings.
3. Enable the "Custom Styling" settings on the trunk branch, switch to PR this branch, and confirm that they continue to work until explicitly disabled.
4. Enable the "Remove Mailchimp CSS" settings on the trunk branch, switch to this PR branch, and confirm that they continue to work until disabled.
5. Perform overall smoke testing to ensure that the plugin works as expected.

Changelog Entry

Changed - Updated the Mailchimp list subscribe form block to include settings for selecting an audience list, reordering fields, toggling field and group visibility, and various other settings.
Removed - The "CSS Cheat Sheet" section from the Mailchimp settings page.
Removed - The "Use JavaScript Support?" option from the Mailchimp settings page.
Removed - The "Use JavaScript Datepicker?" option from the Mailchimp settings page.
Removed - The "Custom Styling" settings from the Mailchimp settings page.
Removed - The "Remove Mailchimp CSS" settings from the Mailchimp settings page.

Credits

Props @jeffpaul @dkotter @iamdharmesh

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.