title: Create custom button(s) for CKeditor 5 toolbar
body: <p>I've managed to customize the header and highlight dropdowns for <a href="https://github.com/ckeditor/ckeditor5-build-classic" rel="nofollow noreferrer">CKEditor 5 – classic editor build</a> by creating a new <a href="https://ckeditor.com/docs/ckeditor5/latest/builds/guides/development/custom-builds.html" rel="nofollow noreferrer">custom build</a>.<br>
  But I don't know how to add additional buttons to the toolbar. For example a fullscreen button.
  Current progress: <a href="https://github.com/barnatekse/ckeditor5-build-lateral/tree/stable" rel="nofollow noreferrer">github link</a></p>