[PoC] Use HTMX for (some?) ajax views

Problem/Motivation

Use HTMX to provide ajax pagination and exposed filters.

Steps to reproduce

Create an ajax view, use htmx for the implementation.

Proposed resolution

Make views work with htmx.

There are many out of scope changes because I tried a few things. There are things I'm doing in the wrong place but I want it to work and let people who know the subsystem tell me how to do it better.

A new feature is that we can get the URL for the HTMX request dynamically. if I have a link with a known href, instead of duplicating the URL, I can do

<a href="/some/url" data-hx-get="attr(href)">link</a>
// this will submit to 
"/some/url"

and that will tell htmx to submit to the url in href, works for action attribute or any other attribute value on the element where data-hx-get is.

We can also add some query parameters (especially to set the _wrapper_format), by adding the query string after a space

<a href="/some/url" data-hx-get="attr(href) ?_wrapper_format=drupal_htmx">link</a>
// In the request this URL would be used.
"/some/url?_wrapper_format=drupal_htmx"

// works with additional params:
<a href="/some/url?filter=data" data-hx-get="attr(href) ?_wrapper_format=drupal_htmx">link</a>
// resulting URL is
"/some/url?filter=data&_wrapper_format=drupal_htmx"

About the syntax I started with [href] but took inspiration from CSS attr(), that can't really be used from JS but the syntax should be familiar to some.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet