src: make a Environment-independent proxy class for NativeModuleLoader

This patch splits `NativeModuleLoader` into two parts - a singleton
that only relies on v8 and `node::Mutex` and a proxy class for
the singleton (`NativeModuleEnv`) that provides limited access to
the singleton as well as C++ bindings for the Node.js binary.
`NativeModuleLoader` is then no longer aware of `Environment`.

PR-URL: https://github.com/nodejs/node/pull/27160


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>