process: move environment variable proxy code into node_env_var.cc

Instead of exposing all the NamedPropertyHandlerConfiguration()
parameters in node_internals, simply expose a CreateEnvVarProxy()
method that returns a Local<Value> that implements process.env,
and mark all the property handlers static in node_env_var.cc.
This makes the code more encapsulated.

PR-URL: https://github.com/nodejs/node/pull/25067


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>