repl: remove REPLServer.createContext side effects

The internal method `REPLServer.createContext()` had
unexpected side effects. When called, the value for the
`underscoreAssigned` and `lines` properties were reset.

This change ensures that those properties are not modified
when a context is created.

Fixes: https://github.com/nodejs/node/issues/14226
Refs: https://github.com/nodejs/node/issues/7619

PR-URL: https://github.com/nodejs/node/pull/14331


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