Emoji email filter doesn't check the content type

[pento]

The emoji email filter needs to check the content type of the email, and only transform emoji into images if it's text/html.

We need to check both $mail['headers'] and the result of the wp_mail_content_type filter.

[pento]

In 31860:

Emoji: When we're replacing emoji with <img>s in email, we can only do that if the Content-Type is text/html - otherwise, they'll show up in the email as the HTML string.

Fixes #31720