repl: deprecate turnOffEditorMode

This deprecates the current REPLServer.prototype.turnOffEditorMode
and adds a private function for turnOffEditorMode which handles the
necessary internal changes required instead of having them scattered
about.

PR-URL: https://github.com/nodejs/node/pull/15136


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>