Proposal: REPL .editor command should open an actual editor and not simple, no-readline stdin

The current REPL responds to .editor by reading stdin like running `cat` with no arguments. It would be a ton friendlier to open an editor like nano or vim and possibly respect the VISUAL and EDITOR environment variables. Alternatively, if the prompt supported multiline readline support like other REPLs, that would be nice, too.

CoffeeScript version 2.3.1
Node: v10.4.0

Start by locating the CoffeeScript REPL implementation and the handling for the `.editor` command, then inspect how it currently reads stdin. Clarify whether completion means launching an external editor using VISUAL or EDITOR, or adding multiline readline support; verify that the selected behavior replaces the current cat-like stdin handling.

Written by the indexing model from the issue text.