src: move `Environment` ctor/dtor into env.cc

This makes it easier to use methods from other headers
in the constructor and destructor.

PR-URL: https://github.com/nodejs/node/pull/19202


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>