repl: disable blocking completions by default

It’s not okay for the REPL to be blocked for multiple seconds after
entering `require('` because the completion is performing blocking
fs operations on potentially huge directories. Turning the REPL
completion function asynchronous would be the right thing to do here,
but unfortunately the way the code is structured doesn’t play well
with that (in particular, it breaks the preview feature).
Therefore, disable these blocking calls by default.

Refs: https://github.com/nodejs/node/pull/33282#issuecomment-733646794

PR-URL: https://github.com/nodejs/node/pull/36564


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>