meta: add explicit deprecation and semver-major policy

* Formalizes deprecation policy
* Introduces End-of-life deprecation phase to identify code to be removed
* Outlines basics of internal vs. public API surface

PR-URL: https://github.com/nodejs/node/pull/7964


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>