/export command appends timestamp to filename, can overwrite output file

[JasonWeill]

The /export command (#658) appends a timestamp, precise to the minute, to the specified filename. For example, when I run /export foo, I get a file named foo-2024-06-21-09-34.md. If I run the same command again within the same minute, the target file is overwritten.

I consider this a bug for two reasons:

1. I didn't expect my filename to have a timestamp appended to it, and there is no option to disable this. Most other features in JupyterLab that save or create files do not append a timestamp to a specified filename.
2. If the target filename already exists, I would expect to see a warning, with an option (e.g., /export -f, for "force") to overwrite the existing file.

See also #852, another recent /export bug.