Feature: Support YJS Subdocs with Collaboration

## Description
Currently, it seems that the `LexicalCollaborationPlugin` makes a lot of assumptions about there only being a single YJS doc and it being set at the root. When trying to use subdocs and only pass a subdoc as the primary doc to be used with lexical collaboration, things don't seem to quite sync correctly.

Start by reading the LexicalCollaborationPlugin implementation and its handling of the primary YJS document. Reproduce the issue with a YJS subdoc passed as the primary document, then trace where root-document assumptions prevent synchronization. Done means collaboration syncs correctly when using subdocs rather than only a single root document.

Written by the indexing model from the issue text.