tty: refactor exports

This commit moves the tty module's exports to a single object,
which is more aligned with other core modules.

PR-URL: https://github.com/nodejs/node/pull/16959


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>