feat(web): add accelerated checkout element shell

[rsomlette]

Review purpose

Establish the smallest buildable and testable home for the Headless accelerated-checkout custom element. This PR contains no merchant configuration or wallet runtime behavior.

Changes

• add the @shopify/checkout-kit/wallets package subpath
• add the wallets.js build entry and declaration output
• synchronously register <shopify-accelerated-checkout-buttons>
• render an empty accessible shadow-root mount
• cover registration, duplicate evaluation, and the empty mount
• update the package snapshot for the new published entry

Non-goals

• merchant inputs or callbacks
• Portable Wallets loading
• wallet eligibility or rendering
• skeleton markup or styling
• local proxy configuration

Related ticket

Related: https://github.com/shop/issues-checkout/issues/14751

This is the shell portion of the ticket. The local development fixture is the next stacked PR, so this PR does not close the ticket.

Validation

• web lint and typecheck
• 260 web tests
• production package build
• publint
• package snapshot comparison