n-api: directly create Local from Persistent

The `v8::PersistentBase<T>.Get` method didn't exist in node 4 and it's
just a helper which creates a new `v8::Local` from the given object.

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/14211


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>