[Facets 3] Render using theme input and select instead of lists with links for checkboxes and dropdown

Problem/Motivation

Currently the facets are always rendered as lists with links, which are rewritten by JavaScript to form elements.

A few issues arise:

• Bots are increasingly aggressively following any links, causing DDOS-like situations, due to heavy traffic. Switching from links to checkboxes may prevent this (added Aug 2025).
• In some themes JavaScript is loaded from the footer, some flickering may arise.
• Slight lag of course because JavaScript has to do it's thing to enable checkboxes and drop-downs
• Styling issues, because the form elements are not rendered by the theme functions of these elements, the html is different and it has to be styled and fixed separately. For a lot of frontenders this could be found annoying.

What did I do?

• Rewritten the DropdownWidget and CheckboxWidget to render the elements server-side using the theme functions for select and checkbox (input__checkbox).
• Rewritten the JavaScript just to trigger on change events for checkboxes and facets.
• Also fixed that theme suggestions are correctly added for the available widget types.
• Added a twig file for checkboxes, because for this element we do not need the (-) element any more when it is active.

See also #3528483: Add missing classes to checkbox widget so it renders correctly.

Steps to reproduce

Proposed resolution

Remaining tasks

Fix hierarchy support for the Checkbox and Dropdown widgets, since taxonomy hierarchy displays break. A structure like this:

- Term 1 (weight 0)
  - Term 1.1 (weight 0)
    - Term 1.1.1 (weight 0)
  - Term 1.2 (weight 1)
- Term 2 (weight 1)
- Term 3 (weight 2)
  - Term 3.1 (weight 0)

This turns into the following (or worse):

- Term 1 (weight 0)
- Term 1.1 (weight 0)
- Term 1.1.1 (weight 0)
- Term 3.1 (weight 0)
- Term 2 (weight 1)
- Term 1.2 (weight 1)
- Term 3 (weight 2)

User interface changes

API changes

Data model changes