Add optional support for Choices.js to the Webform module.

Problem/Motivation

Both the Chosen and Select2 javascript libraries have known accessibility issues. The Webform module and the Drupal community needs to implement a better UX for tagging and multiple select elements.

@see #2346973: Improve usability, accessibility, and scalability of long select lists

Proposed resolution

Add optional support for Choices.js to the Webform module.

Choices.js is a lightweight, configurable select box/text input plugin. Similar to Select2 and Selectize but without the jQuery dependency and better accessibility.

Remaining tasks

Tasks should mirror #2871606: Add (optional) support for Chosen

• Add choices.js to webform.libraries.yml
• Add #choices property support for select elements.
• Create webform.element.choices.js
• Add choices.js test webform
• Add examples

Notes

• Select2 will be recommended default library.
• Choices integration will be very basic.

User interface changes

Choices.js will now be added to libraries.

API changes

None

Data model changes

N/A

Release notes snippet

TDB