repl: deprecate REPLServer.prototype.memory

This method is only useful for the internal mechanics of the REPLServer
and does not need to be exposed in user space. It was previously not
documented, so I believe a Runtime deprecation makes sense.

PR-URL: https://github.com/nodejs/node/pull/16242


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Jeremiah Senkpiel <fishrock123@rocketmail.com>