npm i markdown-it-custom-attrs

import mdItCustomAttrs  from 'markdown-it-custom-attrs'

import mdItCustomAttrs  from 'markdown-it-custom-attrs'
export default {
    markdown:{
    config: (md) => {
        // use more markdown-it plugins!
        md.use(mdItCustomAttrs, 'image', {
            'data-fancybox': "gallery"
        })
        }
    }
}

export default {
    head:[
        [
            "link",
            { rel: "stylesheet", href: "https://cdn.jsdelivr.net/npm/@fancyapps/ui/dist/fancybox.css" },
        ],
        ["script", { src: "https://cdn.jsdelivr.net/npm/@fancyapps/ui@4.0/dist/fancybox.umd.js" }],
    ]
}