Auto-detect if image block term meta binding should be interpreted as an attachment ID or URL

[cr0ybot]

I'd like to give the user the ability to select term meta corresponding to an image in #4, but requiring them to know if the meta value is saved as an attachment ID or not might be asking too much. Instead, we should attempt to auto-detect the meta value format:

1. If value is numeric, try wp_get_attachment_image_url()
2. If value is a URL, use it directly

What other ways could an image be stored in term meta?