Spun off from a discussion at #637 (comment)

From @paulbouwer:

Just want to add my thoughts from a discussion earlier today. I still think it would be nice to be able to use a REPL command (#612) to output to a file.

The scenario I'm thinking of would be if you have just done some work in the REPL but haven't started the REPL with output f.log. Then you could just type something like :output f.log in the REPL.

This could even be extended to:
:output commands f.log - output only commands issued in REPL to the log file
:output outputs f.log - output only output entries to the log file

Not sure if this could leverage this existing PR as a starting point. My only concern with this REPL command approach is that everything will have to be held in some form of in-memory collection by default even if the user has no intention of logging to file. Not sure if this is an issue - there is unlikely to be enough in the collection to cause memory pressure ... famous last words