Add theme compatibility

[jeffpaul]

Alongside #1, we'll also want to test that any styling needs for plugin functionality to render properly on the frontend of sites that we'll test against and add specific compatibility for the following:

• Twenty Twenty-Four default theme: https://wordpress.org/themes/twentytwentyfour/
• "classic", non-block theme, e.g. https://wordpress.org/themes/twentytwentyone/

[dkotter]

In looking through the plugin, seems to support three display options:

1. Added as a widget
2. Added as a shortcode
3. Directly rendered using the mailchimpSF_signup_form function

We'll want to test all three of these and ensure they display properly. Note that block themes (like Twenty Twenty-Four) don't support classic widgets anymore so we'll need to test that on an older theme like Twenty Twenty-One. Should be able to test the other two approaches on both theme types.

[noraconley]

As discussed today, we will also add a small block to work with block themes in a more block editor friendly way (in addition to also working as a shortcode block).

[noraconley]

Nearly complete here. No large issues uncovered, but a couple of small opportunities for improvement. Will have a PR early in the week.

cc @vikrampm1

[noraconley]

I used Contact Form 7 as a sort of benchmark for theme compatibility, as it is the most popular form plugin in the .org directory to the best of my knowledge.

I wanted to make sure that we did not add extra styles that might interfere with the theme or any added custom styles or settings.

The issue that I found is that older themes (<= 2021) did not have full stylesheets added to the block editor. The legacy widget block uses an iframe for “server side rendering”. While I did explore this option, this did not feel like the right approach for consistency and maintainability. A solution I did find was to implement no SSR for a specific deny list of themes (#38). The alternative would be to leave SSR and accept that styles may not be 1:1 between the editor and frontend with older themes.

Tested Successfully:

Twenty Twenty-Four

• Shortcode
• Block

Twenty Twenty-Three

• Shortcode
• Block

Twenty Twenty-Two

• Shortcode
• Block

Twenty Twenty-One (classic editor and widget tested here)

• Shortcode
• Block Classic Editor Shortcake
• Widget