Create attachments via text selection dragging

[andrii-i]

PR #248 adds ability to create attachment from notebook cells or filebrowser files via dragging them into the chat window. Consider how / if text selection will work.

If we don't plan to implement creation of attachments from text selection via dragging, consider removing the selection fields from the IAttachment models as they won't be used (introduced in #247).

I checked on this and I couldn't find a reference for where text selections are handled by JupyterLab (searching for mimedata.setData()). This may actually be native behavior from the browser, since text drags seems to be handled as "cut and paste" by default in browsers.

I think this is OK. Since modern LLM token limits are higher, users should attach the entire file/cell to maximize the context the LLM receives anyways. The persona may be expected to intelligently handle large files by running a grep call first to see which lines in a large file are relevant.

If we're able to reach consensus that this behavior is OK, we should open a follow-up issue to remove the selection fields from the IAttachment models, since they will not be used.

Originally posted by @dlqqq in #248 (review)

[ellisonbg]

I think it would be very helpful to support dragging a text selection to the chat. I have found myself wanting this on a number of occasions.