Add option to open/close PRs based on PR comment

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Problem

As discussed in jupyter-server/jupyter_releaser#521 and jupyterlab/jupyterlab#13505, we would like for the bot to be able to close and open the PR based on a comment. This would allow actions triggered by a workflow to run or re-run the PR workflows, without requiring a personal access token.

Proposed Solution

Add a PR comment listener that will close and reopen the PR based on the comment. The comment by default would be @jupyterlab-bot, please restart ci, but would be configurable with a setting. Note that we own https://github.com/jupyterlab-bot, so we wouldn't be pinging anyone else.

Additional context

This is similar to @conda-forge-admin, please restart ci

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cc @jtpio @fcollonval

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this is a test comment