src: add support for nogc types via `NogcEnv`

[KevinEady]

• Introduce Napi::NogcEnv class, taking the place of Napi::Env in finalizer callbacks.
• Introduce void Napi::NogcEnv::AddPostFinalizer() and overloads.
• Modify tests to support above changes

TODO:

• [ ] docs
• [ ] discuss implementation, namings, etc
• [x] wait for #1409 to land, and incorporate changes