n-api: make per-Context-ness of napi_env explicit

Because instances of `napi_env` are created on a per-global-object
basis and because since most N-API functions refer to builtin JS
objects, `napi_env` is essentially in 1:1 correspondence with
`v8::Context`.

This was not clear from the implementation by itself, but has
emerged from conversations with the N-API team.

This patch changes the `napi_env` implementation to:

- Actually store the `v8::Context` it represents.
- Provide more direct access to the `node::Environment`
  to which the `Context` belongs.
- Do not store the `uv_loop_t*` explicitly, since it can be
  inferred from the `node::Environment` and we actually
  have an N-API method for that.
- Replace calls to `isolate->GetCurrentContext()` with
  the more appropriate `napi_env` `Context`.
- Implement a better (although not perfect) way of cleaning
  up `napi_env` instances.

PR-URL: nodejs#23689
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>