feat: more export types: dataframe, csv, string

A user posted in querychat about exporting the chat history from the shiny application (https://github.com/posit-dev/querychat/issues/34). I ended up putting together code from the Chat.export() method to get the turns and save the text to a markdown file when the user clicks a shiny download button.

If people are going to be building chat applications in Shiny, I totally see the need for a few more export options, mainly being able to save a csv/excel/tabular file. Something that can then be processed in a data analysis pipeline.

Something else that might be useful is if Chat.export() can be refactored so the contents of the file can be accessed separate from writing the file as a side-effect. For example, a separate method to give me the dataframe of the turns/conversation and export saves the dataframe to a file.

In the issue I posed an example where I hobbled together the code for the markdown export type, but if there was a method that already had the string, I would only need to call that method to pass it in as the contexts for a shiny download button.

Start with the existing Chat.export() method and review the context in querychat issue 34. Trace how chat turns are currently collected and written to Markdown, then determine the intended interfaces for string, dataframe, CSV, and Excel output. Done means the requested export representations are available without requiring file-writing side effects.