n-api: Sync with back-compat changes

Background: To enable N-API support for node versions back to v4, the
N-API code can also be built as an external addon. To make maintenance
easier, a single codebase needs to support both built-in and external
scenarios, along with Node versions >= 4 (and corresponding V8
versions).

This change includes several minor fixes to avoid using node
internal APIs and support older V8 versions:
  - Expand node::arraysize
  - In the CHECK_ENV() macro, return an error code instead of calling
    node::FatalError(). This is more consistent with how other invalid
    arguments to N-API functions are handled.
  - In v8impl::SetterCallbackWrapper::SetReturnValue(), do nothing
    instead of calling node::FatalError(). This is more consistent
    with JavaScript setter callbacks, where any returned value is
    silently ignored.
  - When queueing async work items, get the uv default loop instead of
    getting the loop from node::Environment::GetCurrent(). Currently
    that returns the same loop anyway. If/when node supports multiple
    environments, it should have a public API for getting the
    environment & event loop, and we can update this implementation
    then.
  - Use v8::Maybe::FromJust() instead of the newer alias ToChecked()

PR-URL: https://github.com/nodejs/node/pull/12674


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>