Treat the new pmpro-paypal Add On as a no-billing gateway

[dparker1005]

Summary

The new pmpro-paypal Add On (released as 1.1) registers paypal as a checkout gateway and redirects offsite for payment, so no billing address or payment information fields are needed when a member picks PayPal at checkout. Without this change, the donation script's show/hide logic doesn't recognize the paypal slug, so it leaves #pmpro_billing_address_fields and #pmpro_payment_information_fields visible while the user is being redirected offsite.

This adds paypal to both:

• the PHP-side initial value of pmpro_gateway_billing (includes/checkout.php:133), so the page-load state is correct when ?gateway=paypal is set or when paypal is the primary gateway, and
• the JS no_billing_gateways array (includes/checkout.php:186), so picking the PayPal radio toggles the fields off correctly at runtime.

The existing 'check' !== gateway carve-out is unaffected — when the user picks PayPal, we fall into that branch and both billing/payment field groups get hidden, which is the desired behavior for an offsite redirect.

Test plan

• Activate Paid Memberships Pro, the pmpro-donations Add On, and the pmpro-paypal Add On (>= 1.1).
• Configure a paid membership level with the donation field enabled.
• With Stripe (or any onsite gateway) as the primary gateway and PayPal credentials saved on pmpro-paypal, visit the checkout page.
• Selecting the PayPal radio hides both #pmpro_billing_address_fields and #pmpro_payment_information_fields. Switching back to the primary radio restores them.
• Entering a donation amount with PayPal selected does not make the billing fields reappear (the donation should still flow through PayPal offsite).
• With paypal as the primary gateway, the donation field still works and billing/payment fields stay hidden on initial load.
• Existing behavior with paypalexpress, twocheckout, check, and paypalstandard is unchanged.

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