Support CMS Rich Text Class Styling

In my CMS I want to define a custom component, give it classes, and use the Westudio Style Panel to apply styles to the classes.

Example: Callout
```
<div class="callout">
    <img src="/note.svg" class="callout-icon">
    <p class="callout-text">Hello World</p>
</div>
```

Then in Webstudio I want to say `.callout` apply a background for example.