n-api: delete callback bundle via reference

We should strive to use weak persistent references consistently
throughout the code, since using `v8::Persistent` directly results
in having to change many sites in the code when the way we use it
changes.

N-API uses `v8impl::Reference` internally when maintaining a weak
persistent reference is necessary. So far, `v8impl::CallbackBundle` was
using `v8::Persistent` directly in order to weakly reference the JS
function backed by a N-API callback.

The change introduced here reduces `v8impl::CallbackBundle` to a simple
structure and uses a `v8impl::Reference` to weakly reference the N-API
callback with which it is associated. The structure is freed by the
`napi_finalize` callback of the `v8impl::Reference`. This brings
N-API use of `v8::Persistent` completely under the `v8impl::Reference`
umbrella, rendering our use of weak references consistent.

PR-URL: https://github.com/nodejs/node/pull/29479


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>