process: add NODE_NO_WARNINGS environment variable

This commit adds support for a NODE_NO_WARNINGS environment
variable, which duplicates the functionality of the --no-warnings
command line flag.

Fixes: https://github.com/nodejs/node/issues/10802
PR-URL: https://github.com/nodejs/node/pull/10842


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>