lib: guard inspector console using process var

Currently the inspector module is always loaded and if it does not
return anything the inspector console setup is skipped.

This commit uses the process.config.variables.v8_enable_inspector
variable to only load the inspector module if it is enabled.

PR-URL: https://github.com/nodejs/node/pull/15008


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>