tty: add getColorDepth function

Right now it is very difficult to determine if a terminal supports
colors or not. This function adds this functionality by detecting
environment variables and checking process.

PR-URL: https://github.com/nodejs/node/pull/17615


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>