A11y: make the payment action gateway tabs keyboard accessible

[robin-the-going-merry]

Follow-up to #3522 / #3524, which fixed the Global Settings > Payments tabs only.

Broken: the gateway buttons (Stripe, Square, PayPal) in a payment form action use the same tab markup but were not covered. Arrow keys, Home/End and Space/Enter do nothing, and every label is a Tab stop.

Where:

• stripe/views/action-settings/gateway-buttons.php:6-44: role="tablist" with label role="tab", fixed tabindex="0", aria-selected set once on page load.
• stripe/js/frmtrans_admin.js:55: updates aria-selected on change, no keyboard handling.

Fix: reuse initTablistKeyboard() / syncTablistState() from js/src/admin/admin.js (added in #3524) on each gateway tablist, and give the labels a roving tabindex (0 on the checked one, -1 otherwise), same as payments.php. A form can have more than one payment action, so bind per action tablist. Ids already include the action number (gateway-buttons.php:19).

Depends on #3524 merging first.

Requested by: @Crabcyborg

[vivi-the-going-merry]

PR: #3526. It is stacked on #3524 (base is that branch) and will retarget to master when #3524 merges. DeepSource PHP is red on 3 "variable might not be defined" notes for $gateways/$form_action in the included view; those are set by the caller.