Upstream document reset APIs

[dlqqq]

Description

Currently, there are 2 implementation details in our handling of document resets / out-of-band changes which are exclusive to our package:

1. Our usage of custom close codes (4xxx) to indicate document resets
2. Our addition of a reset() method and resetSignal property on the shared models (YChat, YNotebook, YFile).

We will want to align these APIs with jupyter_collaboration, jupyter_ydoc, and other lower layers of the RTC stack in Project Jupyter.

Context

See comments from @ellisonbg in #161.