Add a HTML-purifier-less visual layout plugin

Problem/Motivation

The stock Visual layout plugin is not HTML-5 compatible. See https://www.drupal.org/project/diff/issues/3226910 ✨ Visual inline diff view strips out HTML5 elements Needs review for a ~ 1.5 year old issue with history.

I'm not convinced that HTMLPurifier is needed, given that Drupal manages its own rendering pipeline in its entirety. Adding another step to the process that may produce an entirely different version of markup than end-users might see doesn't seem ideal.

Proposed resolution

Since 0.1.14, caxy/php-htmldiff has an option to completely turn off HTMLPurifier. I think this is the best approach to the problems faced by #3226910.

Remaining tasks

1. Decide what to name the new layout plugin

   Perhaps Visual Inline (HTML5)?

2. Determine the user-configurable options

   None for a MvP. Any options can be added in follow-up feature requests.

3. Determine how to version-detect that caxy/php-htmldiff supports no-purifier mode.

   Specify a minimum version of caxy/php-htmldiff via composer.json.

4. Open follow-up tasks for making the UX for certain element types better

User interface changes

1. A new layout plugin type will be added.

API changes

None.

Data model changes

None.