Support isSynced in Yjs

[paulgb]

Y.Doc has a field isSynced which corresponds to YSweetProvider's synced property (equivalent to status === STATUS_CONNECTED).

The field is not mentioned in the Yjs docs, but it's probably worth implementing anyway for compatibility. The yjs source says:

This is set to true when the connection provider has successfully synced with a backend.
Note that when using peer-to-peer providers this event may not provide very useful.
Also note that not all providers implement this feature. Provider authors are encouraged to fire
the sync event when the doc has been synced (with true as a parameter) or if connection is
lost (with false as a parameter).

In the meantime, developers can access .synced on the YSweetProvider instance instead of the Y.Doc, which is equivalent. They can also use .hasLocalChanges, which reflects whether changes after the initial connection handshake are synced, not just that the initial handshake is complete as .isSynced implies.