Ctrl+c doesn't work with languages that lack line comments

[zshipko]

I'm not sure how many supported languages don't have line comments, but in OCaml there are only block comments. This means that when toggle_comments is used it puts an empty block comment at the beginning of the line.

Instead of:

(* let a = 1 *)

You end up with

(**) let a = 1

[antoyo]

My PR adding partial support for multi-line comment could probably be used as a starting point to support that.

[the-mikedavis]

This falls under #1505