n-api: reimplement instanceof using V8 API

This uses the v8::Value::InstanceOf API exposed in V8 6.0 and should
be performance neutral, if not improve it. By using V8's API, there
is no longer the need to cache Symbol.hasInstance.

PR-URL: https://github.com/nodejs/node/pull/16143


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>