Consider using ProseMirror for RichTextInput?

While doing research for this [forum topic](https://forums.ankiweb.net/t/using-markdown-to-highlight-text/23621/5), I discovered [ProseMirror](https://prosemirror.net/) - a rich-text editor toolkit made by the same guy that wrote CodeMirror.

It has several very intriguing features that would fit well into Anki:
- WYSIWYM Markdown editing (https://prosemirror.net/examples/markdown/)
- Node views (https://prosemirror.net/docs/ref/#view.NodeView)
- allows inline CodeMirror instances (https://prosemirror.net/examples/codemirror/)
- together with [this plugin](https://benrbray.com/prosemirror-math/), we could replace the buggy MathJax editor with inline [KaTeX](https://katex.org/) (#2106)