src: initialize `Environment` members in class definition

Initialize primitive members of `Environment` in the class definition
for clarity.

PR-URL: https://github.com/nodejs/node/pull/25369


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>