Preserve document_id state when setting notebook source (#338)

* preserve document_id state when setting notebook source

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: apeng-stripe

jupyter_ydoc/ynotebook.py

@@ -250,7 +250,9 @@ def set(self, value: Dict) -> None:
             # clear document
             self._ymeta.clear()
             self._ycells.clear()
-            for key in [k for k in self._ystate.keys() if k not in ("dirty", "path")]:
+            for key in [
+                k for k in self._ystate.keys() if k not in ("dirty", "path", "document_id")
+            ]:
                 del self._ystate[key]
 
             # initialize document