Add implementation for napi_add_finalizer

[bghgary]

See BabylonJS/BabylonNative#1351

[bghgary]

This is a stopgap implementation that needs a more comprehensive implementation for all JS engines once this PR is merged.

Tested manually with macOS with the following code:

auto obj = Napi::ArrayBuffer::New(env, 1000000);
napi_add_finalizer(env, obj, const_cast<char*>("blah"), [](napi_env, void* finalize_data, void*) {
constchar* message = static_cast<constchar*>(finalize_data);
NSLog(@"napi_add_finalizer: callback: %s", message);
}, nullptr, nullptr);

[PolygonalSun]

LGTM