Checklist

• Are you reporting a bug? Use github issues for bug reports and feature requests. For general questions, please use https://discuss.yjs.dev/
• Try to report your issue in the correct repository. Yjs consists of many modules. When in doubt, report it to https://github.com/yjs/yjs/issues/

Describe the bug

This plugin fundamentally does not support marks on nodes that are not text nodes. For example, Tiptap's image node, which is an inline, non-text node: https://github.com/ueberdosis/tiptap/blob/v1/packages/tiptap-extensions/src/nodes/Image.js.

To Reproduce

1. Apply a mark to an inline node. For example, apply a hyperlink (https://github.com/ueberdosis/tiptap/blob/v1/packages/tiptap-extensions/src/marks/Link.js) to an image (https://github.com/ueberdosis/tiptap/blob/v1/packages/tiptap-extensions/src/nodes/Image.js)
2. Be the editor of the document. Observe that the JSON version of the prosemirror doc does contain the expected mark, and that the HTML is rendered with the correct <a></a> wrapping the image node.
3. Be a realtime collaborator on the same document, OR reload the document using the serialized ydoc. Observe that the hyperlink on the image is missing because the mark is not serialized into the ydoc.

Expected behavior

Marks on custom nodes are handled just as attributes on custom nodes are handled.