Implement `to_delta`

[jonvet]

I'm struggling to get a text representation including formatting from a YText. There is to_json but that just returns the raw text without formatting. Is there a plan to implement something like https://github.com/y-crdt/y-crdt/blob/40b7bde92ce5595f734d4b7348565d7e8a360af8/ywasm/src/lib.rs#L2242 ?