src: move IsolateData out of Environment

A follow-up commit is going to make IsolateData creation explicit.
In order for that to work, it needs to move out of Environment.

PR-URL: https://github.com/nodejs/node/pull/7082


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>