Try file_picker package, potentially adding an import/export file feature

For Android, cone currently uses `ACTION_OPEN_DOCUMENT`, and it gives the user a convenient experience with linking a file once, and having the read and append work with the up to date file contents. Eventually I'd like to see if we can port cone to iOS using what seems to be a similar function, `openDocument`.

However, I'm not sure when this iOS step will be done, so I'd like to add an additional feature that might help us port to iOS more quickly, and potentially other platforms. There is an externally maintained package named "file_picker", which I'd like to try, and so I think we would be adding to cone a notion of importing and exporting a file.

For example, on the Android side, I think file_picker is effectively using `ACTION_GET_CONTENT` to read.

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Start by reviewing the existing Android ACTION_OPEN_DOCUMENT flow and the file_picker package documentation linked in the issue. Compare the proposed import/export behavior with the current read and append experience, including the possible iOS openDocument path. Done means a concrete, agreed scope for file picking and import/export across the intended platforms.

Written by the indexing model from the issue text.