This PR introduces a beforeTransactionBuilt event in Y.Doc. This event fires after the transaction is built (after the ydoc.transact() callback finishes) but before the transaction is committed.

The purpose of the beforeTransactionBuilt event is adding modifications to the document in response to other modifications somewhere else in the same transaction. Having the extra modifications in the same transaction ensures proper undo/redo and synchronization. My use-case is updating the document metadata when something changes deep inside the document:

ydoc.on("beforeTransactionBuilt", (transaction) => {
  // Watching only user-made changes
  if (transaction.origin === null) {
    ydoc.getMap("metadata").set("modifiedAt", new Date().toISOString());
  }
});

ydoc.on("beforeObserverCalls", (transaction) => {
  if (transaction.origin === null) {
    ydoc.getMap("metadata").set("modifiedAt", new Date().toISOString());
  }
});

function myTransact(ydoc, f) {
  return ydoc.transact(() => {
    const result = f();

    ydoc.getMap("metadata").set("modifiedAt", new Date().toISOString());

    return result;
  });
}

myTransact(ydoc, () => {
  ydoc.getMap("foo").set("bar", "baz");
});

const originalTransact = ydoc.transact.bind(ydoc);

ydoc.transact = (f, origin) => {
  return originalTransact((transaction, doc) => {
    const result = f(transaction, doc);

    if (transaction.origin === null) {
      ydoc.getMap("metadata").set("modifiedAt", new Date().toISOString());
    }

    return result;
  }, origin);
};

I'm not 100% happy with the beforeTransactionBuilt event name, so I'm open for suggestions.

I'm not sure whether there should be a separate "all" event. According to the source code and my experiments, beforeTransaction and beforeAllTransactions events are always emitted together, so I don't understand how the "all" version should work.