Add Cell Menus for downloading cell values

[asg017]

For every cell, include an dropdown option in the left margin of a cell that allows readers to download the values of a given cell.

For example, if you have a cell that has a value of some JSON array, then the menu option will let you 1) download that JSON as a file or 2) copy that JSON value to your clipboard.

I'd imagine downloading and copying are the only operations you would care about. Maybe there could be some "caching" operation that lets you transform some data then include it in a notebook "pre-compiled" or "pre-rendered", but that would be out of scope for now.

• Not for compiled notebooks
• Should work for unnamed cells as well
• Download type: Text, JSON, CSV, Blobs

Could also have some builtin cells that create "Download as X" buttons for a given data variable. That was the original workflow for observablehq.com notebook for download cells, before the cell menu was updated.