add sripts and style and wait for them async to be loaded

add.js

let add = (tag, url) => new Promise( (resolve) => {
    let el = document.createElement(tag);
    if (tag === 'link') { el.rel = 'stylesheet'; }

    if (tag === 'link') {
        el.href = url;
    }
    else if (tag === 'script') {
        el.src = url;
    }

    el.addEventListener('load', resolve);

    document.head.appendChild(el);
});

Promise.all([
    add('script', '//cdn.ckeditor.com/ckeditor5/35.0.1/classic/ckeditor.js'),
    add('style', '//cdn.ckeditor.com/ckeditor5/35.0.1/classic/ckeditor.css'),
])
.then(async _=> {
let instance = ckEditorInstance = await (window as any).ClassicEditor
    .create(document.querySelector( '#editor' ) );
    
    // update orig textarea on change, for form submit
    instance.model.document.on('change', function(ev) { 
        instance.updateSourceElement(); 
        console.log('args', arguments); 
        self.opts.content = instance.data.get();
    });
});