Improve backdrop_html_to_text()

## Description of the need

Implement fixes to `backdrop_html_to_text()` from https://www.drupal.org/project/drupal/issues/299138.


## Proposed solution

The issue thread has some suggested patches. There's also an implementation in MailSystem `_mailsystem_html_to_text()` that may help inform what improvements to make in core. Anyone who uses MailSystem is effectively already using it.
