feat(modal-checkout): add filter for order button text

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds a filter to allow customizing the text for the button to complete an order in modal checkout.

How to test the changes in this Pull Request:

1. Check out this branch.
2. As a reader, start the following transactions via modal checkout (Donate block or Checkout Button block for non-donations):

• One-time donation
• Recurring donation
• One-time non-donation purchase
• Subscription non-donation purchase

3. Confirm that the order button text displays as expected, unchanged from behavior in master (for any Newspack donation products: "Donate now", or whatever the default button label is for non-donation products).
4. Add some custom code to your test site in a temporary plugin or in your theme's functions.php:

/**
 * Customize the order button text.
 *
 * @param string $text The order button text.
 * @param bool   $is_donation True if the current cart contains a Newspack donation product.
 * @param bool   $is_subscription True if the current cart contains a subscription product.
 * @return string The new text.
 */
function newspack_test_order_button_text( $text, $is_donation, $is_subscription ) {
	if ( $is_donation ) {
		$text = $is_subscription ? 'Custom text for recurring donations' : 'Custom text for one-time donations';
	} else {
		$text = $is_subscription ? 'Custom text for non-donation subscriptions' : 'Custom text for one-time non-donation purchases';
	}

	return $text;
}
\add_filter( 'newspack_blocks_modal_checkout_order_button_text', 'newspack_test_order_button_text', 10, 3 );

5. Repeat step 2, and confirm that your custom text is shown as expected in each case.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[miguelpeixe]

In the RAS-ACC project the logic for order button text has been decoupled so the Donation related text lives in the class-donations.php of newspack-plugin and the modal checkout related change lives in the class-modal-checkout.php of newspack-blocks. This will cause a conflict with the proposed filter when the project epic branch merges into master, since there's no longer donation related code in the modal checkout.

Although I don't mind the new filter, these can already be changed without additional code on our end by adding a filter the same way order_button_text works.

[dkoo]

@miguelpeixe thanks for highlighting. And true that the text can already be customized without a new filter, but I think a new filter could make the customization that much easier for non-engineers now, and also make it easier to implement a UI setting if we want to in the future. Will reopen this against the epic.

After looking a bit more closely at the decoupled code, I changed my mind—I don't think the new filter is needed after all. Thanks!