fix(unique-id): do not attempt to append to y-sync plugin transactions

This is because of a bug that we've had for a while, but I was never able to trace down.

- We set an initial block id for the initial content (always "initialBlockId" when collaboration is detected)
- `y-sync` replaces the document with what it "sees" in the `Y.XmlFragment` we gave it
	- This causes a transaction to replace the initial content (replacing it with an empty document with a `blockContainer` containing an id of `null`)
	- The unique id plugin sees this & attempts to correct the missing id, issuing a new transaction
	- This means there is now a write to the `Y.XmlFragment`\

This write can happen independently to the provider actually synchronizing the content.
Meaning, that the `Y.XmlFragment` and `tr.doc` are out of sync when `y-prosemirror` attempts to [restore the selection](https://github.com/yjs/y-prosemirror/blob/ef35266d660c3cd76a491fde243b0c6bee25d585/src/plugins/sync-plugin.js#L634).

This is ultimately because `y-prosemirror` is listening to the Y.Doc _after_ it already is accepting the change. Which is totally valid, but ProseMirror doesn't offer a great way to keep these values in-sync.

The fix here is to just stop the unique-id extension from attempting to amend any `y-prosemirror` transactions. Ultimately, the remote editor should have already written the correct ID (since to the remote editor, it was the local editor - therefore, the unique-id extension should have run).

Author: nperez0111

packages/core/src/extensions/UniqueID/UniqueID.ts

@@ -130,10 +130,18 @@ const UniqueID = Extension.create({
   addProseMirrorPlugins() {
     let dragSourceElement: any = null;
     let transformPasted = false;
+    let isFirstCollaborationTransaction = true;
     return [
       new Plugin({
         key: new PluginKey("uniqueID"),
         appendTransaction: (transactions, oldState, newState) => {
+          if (
+            isFirstCollaborationTransaction &&
+            transactions.some((tr) => tr.getMeta("y-sync$"))
+          ) {
+            isFirstCollaborationTransaction = false;
+            return;
+          }
           const docChanges =
             transactions.some((transaction) => transaction.docChanged) &&
             !oldState.doc.eq(newState.doc);