[Feature Request] Add `tabIndex` option to configure `tabindex` on the outer container or input

Is your feature request related to a problem? Please describe. Currently, Choices.js automatically creates custom wrapper elements (.choices, .choices__inner, etc.) and handles element focus. However, there is no direct configuration option or clean way to set custom tabindex values (e.g., tabindex="0" or tabindex="-1") on the generated wrapper, search input, or select container.

This causes issues in accessible UI flows—such as custom modal focus traps, roving tabindex navigation in complex forms, or skipping disabled dropdowns.

Describe the solution you'd like Provide a configuration option in Choices initialization options:

const choices = new Choices(element, {
  tabIndex: 2, // or -1, custom integer
});