repl: allow autocompletion for scoped packages

Previously, autocompletion of scoped packages was not supported by the
repl due to not including the `@` character in the regular expression.

PR-URL: https://github.com/nodejs/node/pull/10296


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>