repl: avoid parsing division operator as regex

This improves the heuristic used in multiline-prompt mode to determine
whether a given slash character is at the beginning of a regular
expression.

PR-URL: https://github.com/nodejs/node/pull/10103


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: James M Snell <jasnell@keybase.io>
Fixes: https://github.com/nodejs/node/issues/9300