Image Caption

Add captions to images.

Right now uploading image results into a figure tag

<figure>
  <img src="someImage.jpg" alt="" >
</figure>

We need to add the ability to also add image captions

<figure>
  <img src="someImage.jpg" alt="" >
  <figcaption>Some caption</figcaption>
</figure>

All nodes in the schema are in place. Also, I have registered and initiated a plugin (https://gitlab.coko.foundation/wax/wax-prosemirror/-/blob/master/wax-prosemirror-plugins/src/images/captionPlugin.js)

Implementation can be based on the following example with the necessary changes(check caption plugin is the only thing needed). https://glitch.com/edit/#!/pet-figcaption?path=index.js%3A1%3A0

This should be enough for now to add a caption, till we allow react components to be rendered inside the editing surface.