ywasm: YDoc.transact

[Horusiath]

The idea here is to bring ywasm compatibility with yjs further by providing native doc.transact(callback, origin) instead of current doc.beginTransaction.

This would also let us have more fined control over current ambient transaction context: we essentially could cache current transaction inside of the YDoc, just like yjs does, and reuse it on nested ydoc.transact calls.

// ywasm
#[wasm_bindgen]
pub struct YDoc {
  doc: yrs::Doc,
  current_txn: Option<YTransaction>
}

#[wasm_bindgen]
impl YDoc {
  #[wasm_bindgen]
  pub fn transact(&mut self, callback: js_sys::Function, origin: JsValue) {
    let mut txn = self.current_txn.get_or_insert_with(|| self.doc.transact_mut_with(origin).into());
    callback.call(&self.into())
  }
  
  #[wasm_bindgen(getter)]
  pub fn transaction(&mut self) -> Option<&YTransaction> {
    self.current_txn.as_mut()
  }
}