feat: add CSS class to variation buttons for tracking (#1989)

includes/class-modal-checkout.php

@@ -641,7 +641,7 @@ class="<?php echo esc_attr( "$class_prefix {$class_prefix}__modal-container news
 										<div class="variation"><?php echo esc_html( $variation_name ); ?></div>
 										<form data-product="<?php echo esc_attr( wp_json_encode( $product_data ) ); ?>">
 											<input type="hidden" name="newspack_checkout" value="1" />
-											<button type="submit" class="<?php echo esc_attr( "{$class_prefix}__button {$class_prefix}__button--primary" ); ?>"><?php echo esc_html( self::get_modal_checkout_labels( 'checkout_confirm_variation' ) ); ?></button>
+											<button type="submit" class="<?php echo esc_attr( "{$class_prefix}__button {$class_prefix}__button--primary" ); ?> newspack-modal-checkout-variation-selection"><?php echo esc_html( self::get_modal_checkout_labels( 'checkout_confirm_variation' ) ); ?></button>
 										</form>
 									</li>
 								<?php endforeach; ?>