add support for xmltext diff

[stefan-iki]

Hello,

Thanks for creating and maintaining y-rb project. :clap: I've stumbled upon the same - or similar - problem as described in https://github.com/y-crdt/yrb/issues/155 - in my case however I need to serialize/deserialize from XmlText instead of YText.

Can we copy paste solution from ytext.rs to yxml_text.rs? That's what I did in this pull request. I guess it would serve my needs, although I'm not familiar with rust and yrs/yjs stuff, so... I might have mixed things up. :shrug:

What do you think?

Kind regards,