title: CKEditor 5 - How to insert some HTML (aka. where's the source mode)?
body: <p>I want to be able to switch from WYSIWYG to plain HTML to e.g. insert IFrame with 
  a Youtube video. So far with the standard CKEditor 5 builds there is no documentation on how to do that.</p>
  
  <p>Is there an equivalent of the <a href="https://ckeditor.com/cke4/addon/sourcearea" rel="noreferrer">Source Editing Area</a> plugin but for CKEditor 5?</p>