repl: always check for NODE_REPL_MODE environment variable

This makes sure all REPL instances check for the NODE_REPL_MODE
environment variable in case the `replMode` is not passed through
as option.

At the same time this simplifies the internal REPL code significantly.

Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>

PR-URL: https://github.com/nodejs/node/pull/33461


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>