Ipyflow

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* [kernel] Misc bugfixes;

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* [kernel] Add another comm handler for last_updated_cell_id;

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* [kernel] Add a couple more comm handlers;
* [kernel] Allow reactivity_mode to be configured on client initialization;
* [kernel] Improve compatibility with other interfaces;

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* [lab][kernel] Persist dependency DAG across restarts;
* [lab] Continuous file sync for cell input content changes;
* [lab] Bugfix for clearing [*] in case where intitial cell in reactive chain threw an error;
* [kernel] Improve loop performance;
* [kernel] Improve accuracy by expanding cases for tracing reenablement;
* [kernel] Misc bugfixes;

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* [lab] Add batch reactivity_mode and make it the default (previously was incremental);
* [lab] Add slice indicators to the left of cell counters;
* [lab] Fix bug related to syncing dirty cell indicator to dirty state after paste;
* [lab] Add ctrl / cmd + j, k for execute forward, backward slices, respectively;
* [kernel] Enable debugger support;
* [kernel] Provide a ipyflow-based TerminalInteractiveShell launchable via 'ipyflow' on the command line;
* [kernel] Fix bug wherein tracing wasn't totally disabled in line magics;
* [kernel] Fix bug wherein statement dependencies would not be updated for cells that react to later versions of an updated slider;
* [kernel] Make sure ipython also captures static dependencies;
* [kernel] Various minor bugfixes;

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* Set consistent comm ids when APIs permit;