n-api: restrict exports by version

* Move `napi_get_uv_event_loop` into the `NAPI_VERSION >= 2` section
* Move `napi_open_callback_scope`, `napi_close_callback_scope`,
  `napi_fatal_exception`, `napi_add_env_cleanup_hook`, and
  `napi_remove_env_cleanup_hook` into the `NAPI_VERSION >= 3` section
* Added a missing `added` property to `napi_get_uv_event_loop` in the
  docs
* Added a `napiVersion` property to the docs and updated the parser and
  generator to use it.
* Added usage documentation

PR-URL: https://github.com/nodejs/node/pull/19962


Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>