fix(list-table): port ACF _acf-copyable.js for click-to-copy field keys

[faisalahammad]

Ports upstream ACF 6.8.5's assets/src/js/_acf-copyable.js so the Key column on the Field Groups / Post Types / Taxonomies / Options Pages list tables becomes click-to-copy, matching the existing field-group editor behavior.

The CSS for the hover copy icon and the green check on .copied was already synced from ACF 6.8.5 in #506, but was inert because no JS was emitting the .copyable markup. This PR adds the JS half and wires it into the acf.js entry bundle. The acf script handle is already enqueued on the four edit-acf-* list screens, so no PHP or CSS change is required.

Diff:

• assets/src/js/_acf-copyable.js (new) - acf.Model gated on pagenow matching edit-acf-field-group, edit-acf-post-type, edit-acf-taxonomy, edit-acf-ui-options-page. Finds .column-acf-key cells, wraps the key text in a span.copyable (adds copy-unsupported when navigator.clipboard is unavailable), and handles the copy click.
• assets/src/js/acf.js - added import './_acf-copyable.js'; after the _acf-tooltip.js import, matching upstream's entry ordering.

How to test

1. Build the assets (the bundled acf.js is checked in under assets/build/ and shipped; verify locally with npm run build if needed).
2. On the WP admin, open Field Groups / Post Types / Taxonomies / Options Pages. Make the Key column visible via Screen Options if needed.
3. Hover any row's Key cell - a copy icon now appears next to the key.
4. Click the key - the field group / post type / taxonomy / options page key is copied to the clipboard, and the icon briefly turns into a green check for about two seconds.
5. Open a Field Group for editing and click a field name/key to copy. Same behavior as before, no double-fire or console errors.
6. (Optional) Test over plain HTTP (not localhost/HTTPS): the icon hides because navigator.clipboard is unavailable in non-secure contexts; no JS errors.

Screenshots

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