n-api: unexpose symbols and remove EXTERNAL_NAPI

* namespaced functions such as v8impl::JsHandleScopeFromV8HandleScope
  become part of Node's public symbols unless they are declared static.
* the class uvimpl::Work needs to be enclosed in an anonymous namespace
  else it, too becomes part of Node's public symbols.
* remove references to EXTERNAL_NAPI.

PR-URL: https://github.com/nodejs/node/pull/16234


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>