Refactor big_pipe module to remove dependencies on Ajax classes

From Slack:

Great work! And great work to get the first patch in.

I pondered the messages issue a little bit - from your big refactor and I would suggest do do two template elements.

One is a type=messages stream and one is simple replacements like the big pipe elements you already had.

That way:

• You keep it simple.
• You keep the HTML as the format for data
• Messages stream is several “<messages>” HTML elements (or whatever you want to define), which you parse back to JS and then use the Drupal.message() API (simple 1:1 mapping from HTML to API call -> 1 element for each API call)

That way:

• Everything is template elements
• HTML is the data format
• Messages are added by JS (as we defined they would be)
• BigPipe is still a very simple .swap operation with the placeholder by default

I very much affirm HTML as the data format.

In my last draft I was sending individual messages as HTML but I now think that all we need to do is render the messages normally using '#theme' => 'status_messages'. If the theme provides the selector for the wrapper element, with fallback to core div[data-drupal-messages] we can start get the innerHTML of the incoming wrapper and swap it into the existing wrapper in the DOM using the HTMX beforeend strategy.

Discussed with @nod_ yesterday. He pointed out that we can communicate in a CR that themes need to apply the data-drupal-messages attribute to their wrapper element.