Draft: Resolve #3541597 "Ignore htmx dx"

Test for HTMX backend DX:

instead of using raw attributes use a kind of builder object and apply the result to the render array.

$form['config_name'] = [];
(new Htmx())
  ->post()
  ->swapOob()
  ->swap(HtmxSwapStyle::none)
  ->applyTo($form['config_name']);

When calling applyTo everything is applied as-is to $form['config_name']['#htmx'], so it becomes:

$form['config_name'] = [
  '#htmx' => [
    'post' => TRUE,
    'swap-oob' => TRUE,
    'swap' => ['style' => HtmxSwapStyle::none, 'modifiers' => new HtmxSwapModifiers()],
  ],
];

and this key is picked up in Renderer::render just like #states, and is finally transformed by HtmxHelper::processHtmx and results in:

$form['config_name'] = [
  '#attributes' => [
    'data-hx-post' => '',
    'data-hx-swap-oob' => 'true',
    'data-hx-swap' => 'none',
  ],
];