add support for conversational state to assistance endpoint (#506)

* add support for conversational state to assistance endpoint

This refactors the assistance code somewhat, to allow carrying
along some conversational state. It extends the OpenAI-flavored
assistant to make use of that state to have a conversation.
The front-end is tweaked a little bit to allow for replies that
don't have any code in them (though I didn't get into formatting
such replies nicely).

Currently tested primarily through the runCompletion script,
which has been extended a bit to allow testing simulated
conversations (where an error is pasted in follow-up, or
an expected-vs-actual comparison).

Co-authored-by: George Gevoian <85144792+georgegevoian@users.noreply.github.com>

sandbox/grist/formula_prompt.py

@@ -37,7 +37,6 @@ def column_type(engine, table_id, col_id):
       Attachments="Any",
     )[parts[0]]
 
-
 def choices(col_rec):
   try:
     widget_options = json.loads(col_rec.widgetOptions)