Refactor for broader theme support and simpler, selector-based API

Problem/Motivation

• Currently assumes Vertical and Horizontal Tabs markup and containes theme-specific logic for those layouts.
• USWDS Base and potentially other themes render Field Group tabs as accordions with different markup (no <details>, custom classes, etc.), which caused Field Group Complete to fail to attach any behavior at all.
• The previous approach also introduced a more complex PHP/JS contract (pane → tab mapping, hooks, and handler concepts) that proved unnecessary once we discovered a robust, universal selector.

Proposed resolution

Refactor Field Group Complete’s JS and form alter to:

• Use a universal pane selector that targets the Field Group tabs container in a theme-agnostic way.
• Use a single, generalized trigger selector for tab labels/buttons:
  

  const DEFAULT_TRIGGER_SELECTOR =
    '.field-group-tabs-wrapper a[href^="#edit-group-"], ' +
    '.field-group-tabs-wrapper button[aria-controls^="edit-group-"]';
  

• Keep the logic for mapping “pane → trigger” fully client-side and markup-driven, without theme-specific PHP hooks.

Split the JavaScript into:

• js/field_group_complete.core.js: core logic (required detection, completeness computation, badge creation, class toggling, etc.).
• js/field_group_complete.js: the Drupal behavior (initialization, selector discovery, and calling into the core functions).

Retain vertical/horizontal tabs support by:

• Continuing to look up triggers inside the same .field-group-tabs-wrapper container.
• Preserving compatibility with Vertical Tabs’ .vertical-tabs__menu-link-content as an optional inner label wrapper when present.

Remove the experimental pane ↔ tab mapping hooks and handler APIs introduced earlier, since they are no longer necessary with the universal selector approach.