index-main/site/plugins/horizontal-gallery/index.php

<?php

Kirby::plugin('index/horizontal-gallery', [
    'blueprints' => [
        'blocks/horizontal-gallery' => __DIR__ . '/blueprints/blocks/horizontal-gallery.yml'
    ],
    'snippets' => [
        'blocks/horizontal-gallery' => __DIR__ . '/snippets/blocks/horizontal-gallery.php'
    ]
]);